Manchester City, Cristiano Ronaldo, Lionel Messi, Ajax, RB Leipzig and Liverpool (from left to right top to bottom)

Will an English side win the Champions League again? What shape are their La Liga rivals in? Could Ajax repeat last season's heroics?

European football's premier competition returns this week with some fixtures worthy of the knockout stages.

On Tuesday, holders Liverpool go to Napoli, Chelsea welcome Valencia and Borussia Dortmund host Barcelona, while on Wednesday, Juventus travel to Atletico Madrid and Paris St-Germain host Real Madrid.

BBC Radio 5 Live will have commentary of Napoli v Liverpool, then Olympiakos v Tottenham and Shakhtar Donestk v Manchester City on Wednesday.

Can an English team win it again?

Liverpool and Tottenham met in last season's all-English final

Two of the three favourites to win this season's tournament are English - holders Liverpool and Premier League champions Manchester City.

Jurgen Klopp's Liverpool are hoping to do what only one team have managed since the competition was rebranded as the Champions League in 1992 - retain the trophy.

Real Madrid's 'three-peat' - when they won in 2016, 2017 and 2018 - was the first time a team had won consecutive European Cups since AC Milan in 1989 and 1990.

Consistency could be the key. Real only named 13 starters across their three finals. Premier League leaders Liverpool still have 10 of their starting XI from the 2018 final defeat by Real, with only goalkeeper Loris Karius having departed, and could conceivably name last season's final line-up on a regular basis this season.

Manchester City are the bookmakers' favourites to win Europe's elite competition for the first time.

They look in great shape (albeit perhaps slightly less so after Saturday's shock defeat by Norwich - only their second loss since January). Pep Guardiola's side won the clean sweep of English trophies last season, and have picked up 198 points in their past two full Premier League seasons.

They have added to their squad, with Rodri and Joao Cancelo, while star midfielder Kevin de Bruyne has returned after an injury-hit season.

Add to that the fact Raheem Sterling and Sergio Aguero are scoring a goal a game or more this season and they surely have a big chance of lifting the trophy in Istanbul on 30 May 2020.

"When you speak to top Spanish clubs, top Italian clubs, top French clubs, they're all pretty unanimous that English clubs are the ones to fear in the Champions League," football journalist James Horncastle said on last week's BBC Radio 5 Live Football Daily.

Tottenham will be hoping to go one better than last season, when they lost to Liverpool in the final.

They reached that stage despite not having signed a player all season and losing striker Harry Kane to injury for key games.

Spurs have strengthened since then, but they have had an up-and-down start to this campaign, with two wins from five Premier League games.

Frank Lampard will be a Champions League manager for the first time as a youthful Chelsea return to Europe's top table after a year away, during which time they won the Europa League.

Lampard, a 2012 European champion as a Blues player, replaced Maurizio Sarri this summer and, with the club under a transfer embargo, has looked to youth. All 11 of their Premier League goals this season have been scored by English youngsters who were on loan in the Championship last season - Tammy Abraham, who was at Aston Villa, and Mason Mount and Fikayo Tomori, who were with Lampard at Derby.

How are La Liga's big three looking?

Frenkie de Jong, Ansu Fati and Antoine Griezmann (all pictured with Gerard Pique, second right) give Barcelona a new look this season

Barcelona are the second favourites to win the tournament, but the Spanish champions' past two European titles were 2011 and 2015.

Both of Ernesto Valverde's seasons in charge have ended with a Champions League implosion after a three-goal win - against Roma in the 2018 quarter-finals and then against Liverpool in the semi-finals earlier this year.

He has freshened things up, with the arrival of forward Antoine Griezmann from Atletico Madrid and midfielder Frenkie de Jong from Ajax, as well as the emergence of wonderkid Ansu Fati. Superstar Lionel Messi is yet to play this season after injury, but he is in the squad for their opening game against Borussia Dortmund.

"He's 32 and for the first time ever I have started hearing stories of people close to Messi saying: 'How about if he has only got two more years at Barcelona,'" Spanish journalist Guillem Balague said. "I went into panic... only two more years of Messi?

"We haven't seen the best of Barcelona yet this season. It's been a poor start but there is still time to get that corrected."

Barcelona's eternal rivals Real Madrid do not look in the best of shape so far. Zinedine Zidane is looking to win the Champions League for a fourth time as Real manager - he stepped down last summer after three in a row but came back in March, after their European exit.

However, his rebuild of the club has so far not materialised as planned. They spent over £200m on five players this summer, but Zidane had hoped for more - including midfielder Paul Pogba from Manchester United - and he was unable to move on some players he did not want, including forward Gareth Bale.

In fact, none of their new signings started in Saturday's 3-2 win over Levante, although Eden Hazard came off the bench for his debut.

"It must be a must-win month," said Balague. "If things don't go well in September we may be seeing a new coach, certainly by Christmas.

"It's not just about results. Things have not worked as Zidane expected, and things are not working the way [president] Florentino Perez expected.

"There was going to be a revolution, Zidane said that himself last season. Instead, we have seen the same players. Out of all the new ones, only Eden Hazard will go straight into the line-up. Everybody else will be on the bench."

A lot of observers are fascinated to know how the new-look Atletico Madrid get on this season.

Diego Simeone spent over £200m this summer and recouped even more in fees. Among the players to come in were Benfica forward Joao Felix (more on him later), Tottenham right-back Kieran Trippier, Real Madrid midfielder Marcos Llorente and Espanyol defender Mario Hermoso.

Three of their famous back four left on free transfers - Diego Godin, Filipe Luis and Juanfran - plus Griezmann to Barca, Rodri to Manchester City and Lucas Hernandez to Bayern Munich.

It is a decade since Atletico, who have never won the Champions League, have been eliminated in the knockout stages to a team not including Cristiano Ronaldo, who is in this season's group stages with them. City rivals Real beat them in four consecutive years, including two finals. And last year Ronaldo scored a hat-trick as Juventus overturned a 2-0 deficit in the last 16. That ended up being a big moment.

Balague said: "It had a bearing on the future and present of Atletico. For Simeone, that was confirmation the team needed to get rid of players like Godin and Juanfran. Now he's got an exciting, very versatile team."

Valencia, who play Chelsea in the opening game, seem to be going through a now-traditional self-destruction. Just as things were looking good, they sacked the manager who got them into the Champions League and led them to the Copa del Rey last season - replacing Marcelino with Albert Celades last week.

How about the other contenders?

Will Neymar and PSG finally deliver in Europe this season?

There are three other teams expected to compete for the trophy, all perennial champions of their countries - Juventus (eight Italian titles in a row), Bayern Munich (seven consecutive German titles) and Paris St-Germain (six of the past seven French leagues).

Juventus - whose last Champions League title came in 1996 - have added youth and dynamism to their squad, with £67.5m Netherlands centre-back Matthijs de Ligt the most notable recruit. That signing was especially important, following the loss of captain Giorgio Chiellini to a long-term knee injury.

Three free transfers came in - Arsenal and Wales midfielder Aaron Ramsey, Paris St-Germain and France midfielder Adrien Rabiot and goalkeeper Gianluigi Buffon, a Juventus legend who only left a year ago for PSG. And of course they have the Champions League's greatest goalscorer in Ronaldo.

Their squad is so large that midfielder Emre Can and forward Mario Mandzukic have been left out of the 22-man Champions League selection completely.

Horncastle said: "Sarri said the squad was too big in the summer. He said: 'We have got six players too many and we have to sell players.' They were unable to. This is on the club - not Sarri. They went bullish at the start of the transfer window and could not sell Paulo Dybala or Mandzukic. It's been a real problems for them."

Bayern Munich, who host Red Star Belgrade on Wednesday, go into a season without Arjen Robben and Franck Ribery on the wings (or in the treatment room) for the first time in a decade. For a while it looked as if they would not sign replacements, but Philippe Coutinho and Ivan Perisic have boosted their options.

French World Cup-winning full-backs Hernandez and Benjamin Pavard were their other two big-money signings, but the expected overhaul did not quite materialise.

Where to start with Paris St-Germain? Is this the season they finally get their act together in Europe? That is what people have been wondering for about seven years.

The French giants look no closer to achieving their owners' goal of being crowned European champions and seem to be going backwards. They have gone out in the last 16 for the past three years, having reached four consecutive quarter-finals before that.

They are without Neymar, who failed to get a move to Barcelona this summer, for half of the group stage because of a ban following his reaction to last year's collapse against Manchester United. He was booed relentlessly on his return to the team at the weekend.

But they have added forward Mauro Icardi to their frontline this summer, as well as finally investing in holding midfielders - Ander Herrera and Idrissa Gueye.

Which players could light up the Champions League?

Raheem Sterling could have a big season ahead

The two top scorers in Champions League history, Juventus forward Cristiano Ronaldo (126) and Barcelona captain Lionel Messi (112), go without saying every year in the list of potential stars. Between them, the pair have won, or shared, the Golden Boot in each of the past 12 seasons.

Elsewhere, this could be a big season for Atletico Madrid's new signing Joao Felix. They signed the 19-year-old from Benfica for £113m and he has already shown some real touches of quality in pre-season and the opening weeks.

The Portuguese only made his professional debut last August, became the youngest hat-trick scorer in the Europa League and scored 15 league goals as Benfica won the domestic title.

He was effectively Atletico's replacement for Griezmann, despite signing before the Frenchman left for Barcelona.

Neymar needs a big season in the Champions League to boost his reputation and maybe win the PSG fans back. Both of his previous seasons in Paris have ended early through injury, and he is in danger of gaining a flat-track bully reputation in France, if he does not already have it.

PSG team-mate and perennial Real Madrid target Kylian Mbappe could have a big season ahead of him. It could be the French World Cup winner's final season at his hometown club if media reports are to be believed. The 20-year-old scored 33 goals in 29 Ligue 1 games last season, but has managed only one Champions League knockout goal for the club in two seasons.

Elsewhere, Manchester City winger Raheem Sterling's form this season makes him look a genuine candidate to be among the best players in this season's tournament. Boss Guardiola says he could be capable of reaching Messi or Ronaldo level.

How are Ajax faring?

Dusan Tadic (right) and Ajax have carried over their blistering 2018-19 form into this season. Tadic has scored in his past eight league games.

Ajax's heartbreaking last-second semi-final defeat by Tottenham felt like the end of an era. Unlike the collapses of Barcelona or PSG in the past, it never seemed like this side were going to have a chance to go again the following season.

Much like Monaco's Ligue 1 title-winning squad in 2017, Ajax's team was always destined to be gutted by Europe's big-spending elite.

De Jong's move to Barcelona in the summer had already been announced, and captain De Ligt - who ended up joining Juventus - was always likely to leave with practically every rich club interested.

Those two were absolutely key to Ajax's success. But there was perhaps not the huge exodus people expected, with Denmark forward Kasper Dolberg's move to Nice the only other sale of £10m or more.

Winger Hakim Ziyech looked set to leave, but has instead extended his contract, and midfielder Donny van de Beek, reportedly a Real Madrid target, has stayed with the club. Dusan Tadic - an unlikely star of last season's surprise run - has also stayed put.

The Dutch champions, who had to come through qualifying rounds again, look like they might have recruited well, with Sevilla winger Quincy Promes their biggest transfer.

They are unbeaten in nine Eredivisie and Champions League qualifiers this season, and host Lille on Tuesday.

Could there be a surprise package this season?

RB Leipzig drew with Bayern Munich on Saturday to stay top of the Bundesliga

If there is to be a surprise package this season, Group G looks a likely source. RB Leipzig, Benfica, Lyon and Zenit St Petersburg, who signed Barcelona winger Malcom this summer, all have exciting players in their squad.

RB Leipzig, who are at Benfica on Tuesday, are never going to appeal to the romantics like Ajax, as they were only formed in 2009 by an energy drinks company. But they have a decent chance of a good run this season.

They have one of the world's most exciting young managers in Julian Nagelsmann, who already has three and a half years of managerial experience with Hoffenheim despite only being 32.

Leipzig have potential superstar Timo Werner up front, and likely future France centre-back Dayot Upamecano in defence. And this summer they signed two British players - long-term target Ademola Lookman on a permanent deal from Everton and Ethan Ampadu on loan from Chelsea. They also brought in midfielder Christopher Nkunku from PSG and went to Roma for forward Patrik Schick.

They are the unbeaten Bundesliga leaders after drawing 1-1 with champions Bayern at the weekend.

Benfica have recruited well after losing Felix to Atletico, with striker Raul de Tomas joining from Real Madrid after two prolific seasons on loan at Rayo Vallecano.

However, Lyon could be weaker than last season, with Tanguy Ndombele joining Spurs, Ferland Mendy going to Real Madrid and captain Nabil Fekir leaving for Real Betis.

Elsewhere, Atalanta - in a group with Manchester City, Shakhtar Donetsk and Wednesday's opponents Dinamo Zagreb, could be a team to keep an eye on.

They are seeded bottom because this is their Champions League debut, but they finished third in Serie A last season - above Inter Milan, AC Milan and Roma.

Gian Piero Gasperini's side were the top scorers in the league with 77, outscoring champions Juventus by seven goals.

Previously unheralded Colombian striker Duvan Zapata was the second top scorer in Serie A last season with 23 goals, while no player set up more than Alejandro Gomez's 11 in the league. Add to that 12-goal Josip Ilicic and new signing Luis Muriel and that is a whole lot of firepower.

Here are all of this week's fixtures...