The Champions League group stage begins with 31 teams attempting to deny Real Madrid a fourth straight triumph. All of the main contenders have questions to answer, so we asked Michael Cox to look at one key issue for each of the likely challengers.

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The major surprise from the European Cup holders' summer was not that Real Madrid sold Cristiano Ronaldo, but that they did not really replace him. Kylian Mbappe, Eden Hazard and Neymar seemed the obvious choices, but instead Madrid are taking the very un-Real approach of expecting others to step up. Fortunately, there is certainly the potential within the ranks for individual development.

Gareth Bale will relish the opportunity to become the main man, while Karim Benzema can focus on scoring himself rather than teeing up Ronaldo and Marco Asensio is one of Europe's most talented youngsters. Isco, meanwhile, might become a key tactical weapon rather than flitting around, almost separated from the tactical system. Ultimately, though, Ronaldo scored a goal a game, so every attacker must improve his goal return to compensate.

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Juventus have regularly reached the latter stages of the Champions League in recent years and were beaten finalists in 2015 and 2017. Their advantage over every other side in the competition is their tactical acumen: their defensive ability combined with the flexibility that comes naturally to Italian sides. But will the presence of Cristiano Ronaldo affect that?

Ronaldo is not particularly keen on taking defensive responsibility and, while he is considered an outright centre-forward rather than a wide player, Juve boss Max Allegri has sometimes asked strikers to retreat into a deep block. That is a relatively unfamiliar role for the ex-Real Madrid man and so, while Juve have more star quality, they have perhaps lost some tactical harmony.

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Liverpool played in May's Champions League final and have improved this season, but their status as Premier League title contenders means that Jurgen Klopp's resources will be stretched, given the Reds will be desperate not to drop any points in their bid to dethrone Manchester City.

That scenario could make it difficult to compete on two fronts, especially as their manager's high-energy system drains legs and there is suspicion it increases the likelihood of injuries. Liverpool have a better squad -- newcomers like Xherdan Shaqiri and Fabinho have been unable to break into the first XI -- but it is notable that Klopp's sides have usually mounted a serious challenge at home or abroad, rather than both.

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We have become so accustomed to the same old names touring Europe's major clubs as manager that, when one of the big sides takes a chance on the boss of a smaller side, it comes as something of a surprise.

Niko Kovac impressed while in charge of Eintracht Frankfurt -- his parting gift in May was a German Cup final win over Bayern -- but expectations are greater at Bayern in terms of tactics and man management. Ultimately, Kovac has not yet been tested against Europe's elite and, as Carlo Ancelotti found last season, the Bayern dressing room can be difficult to win over.

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It felt strange when Xavi Hernandez left Barcelona three years ago and it feels more odd now that his old mate Andres Iniesta has also departed. Although not always a regular last season, he often saved his best performances for big games, dictating the midfield passing with grace and patience.

Without him, Philippe Coutinho provides flashes of inspiration more than metronomic passing quality, while Ivan Rakitic always seems better playing a supporting role rather than leading the midfield. Sergio Busquets operates in a very different role to Iniesta, so it might be down to Lionel Messi to play deeper than ever, starting moves more than finishing them.

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Winning a knockout competition usually depends upon keeping key clean sheets along the way, but Manchester City have never looked entirely convincing at the back under Pep Guardiola. He has created a technical side that plays without a conventional defensive midfielder and has increasingly attempted to use technical centre-backs.

John Stones and Aymeric Laporte have been favoured this season, but Stones remains liable to mistakes and Laporte is a similar operator rather than a defensive rock. City conceded three goals against Liverpool in the first half hour of last season's quarterfinal, essentially putting an end to their Champions League ambitions. They will need to be more solid this time around.

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The obvious question remains the most important factor in assessing Manchester United's likely European performance. Mourinho's "third-season syndrome" caused problems during his time at Real Madrid and Chelsea, and United's form so far this season has been thoroughly unconvincing, with their manager still uncertain about his preferred combinations in midfield and in defence.

Mourinho made his name in this competition, triumphing with Porto in 2004 and winning his second European Cup with Inter in 2010, but nine seasons since his last Champions League success, he must prove he can compete with Europe's best and lead a club for a sustained period.

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Not since Porto beat Monaco in 2004 has a club from outside Spain, England, Germany and Italy reached the Champions League final. PSG might seem the most likely to "break in" but, despite their riches, the French giants have not been able to progress even as far as the semifinals. A major reason for this is that they are simply not tested enough in Ligue 1, even if it does keep them physically fresh.

PSG's domestic games are not only too easy to sustain the requisite level for Champions League winners, but they are also entirely difficult tactically; they always dominate possession and usual have to break down a deep defence. In Europe, when pressed and outpassed and on the back foot, PSG can be exposed.

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It seems bizarre to speculate about a possible decline in Harry Kane's scoring ability, considering last season was his most prolific to date -- he netted 41 goals in all competitions -- and he won the World Cup's Golden Boot in the summer. However, he looks less mobile and sharp and his shot totals have declined significantly: An average of almost five per game in last season's Premier League has dropped to half that number.

There is more to his game than goals and he has played a good supporting role for a strike partner with more energy: Raheem Sterling for England and Lucas Moura at club level. But Spurs have come to depend upon Kane's prolific scoring ability and, if he does not find top form in the spring, it is difficult to imagine them going deep in the Champions League.

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The nature of Antoine Griezmann's decision about where to play this season left a bitter taste, especially given he chose to announce it in a video on social media, but the decision itself was a huge boost to Spanish and European football. Rather than becoming yet another big name at Barcelona, he remains the only true superstar at Atletico.

That is crucial for Diego Simeone's side, an otherwise solid, brilliantly organised team that would lack individual magic without the French forward. Griezmann can play upfront alongside Diego Costa but has increasingly showed an ability to drop deep, orchestrate play and provide penetrative passes. That role might be more important in the Champions League, as Atletico attempt to break down tighter defences than they encounter in La Liga.

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Carlo Ancelotti essentially made his name by succeeding in European competition, taking Milan to two European Cup triumphs before winning another with Real Madrid. Indeed, his sides have often fared better in the Champions League than in domestic league play. However, in recent years Ancelotti's overall tactical acumen has been questioned.

Last season's failure at Bayern Munich, from where he was sacked before the end of September, was particularly notable. His players felt that training sessions lacked the requisite intensity and complexity and there is a sense that Ancelotti remains a laissez-faire manager in an era that demands game plans that are more intricate. Napoli's new manager is seeking to prove he has not been left behind.