Fabinho, Naby Keita and Xherdan Shaqiri were all on the bench against Napoli. The trio cost a combined £100m last summer

There are many different ways to judge Liverpool's development under Jurgen Klopp but I see their squad strength as the best way to gauge their progress, and it will also play a big part in any future success.

Look at the players who did not start their win over Napoli on Tuesday night and you can see the Reds now have true depth as well as a quality first XI.

That is significant because Klopp's style of football is extremely taxing, but it becomes particularly important now they are contenders in the two biggest competitions going into the second half of the season.

It has been a long time since you could say that about Liverpool - the last time they seriously challenged for the league title, in 2014, they were not in Europe at all, and during last season's Champions League run they were nowhere near Manchester City at the top of the Premier League, finishing fourth, 25 points back.

They still almost ran out of players on their way to the final in Kiev in May - when they lost to Real Madrid, they only had three fit midfield players left. If one of them got injured, right-back Trent Alexander-Arnold was next in line.

Things are already different this season - they have successfully married their excellent start in the Premier League with progress in the Champions League, sealing their place in the last 16 by beating the southern Italian side this week.

Their squad has already played a key part in that dual challenge, and it is the biggest reason I think they could go the distance on both fronts.

I am not going to say they will win the Premier League or the Champions League, let alone both, because winning just one will be incredibly tough.

But what I do know is that they will give it a really good go and, with the squad that Klopp has built, they are well equipped this time to build on the position they have put themselves in.

Liverpool could get stronger in January, not weaker

He gets results and his teams are attractive to watch, so of course lots of other clubs would love to have Klopp in charge.

But after more than three years in charge of the Reds, he needs a trophy this season. What they have done so far in this campaign is all very promising, but it counts for nothing unless they cap it with some silverware.

So, it helps that it is not just in terms of their results in Europe or their Premier League position that Liverpool are in a fantastic situation.

Financially, the club is in great shape too. If Klopp wants to buy in January, then he will have the funds to do so.

I find it intriguing that, for the first time, players are no longer cup-tied in Europe if they have already played for one Champions League side this season.

That could be a factor in Liverpool luring a player who could really enhance their squad next month. If they show an interest in someone, he will know he has a decent chance of reaching the later rounds and could be seriously tempted.

Watch: Klopp 'full of adrenaline' after 'unbelievable' Liverpool reach Champions League last 16

But regardless of whether anyone arrives at Anfield in January, it looks extremely unlikely that any of their star players will be leaving mid-season like Philippe Coutinho did last season.

It turned out that Coutinho's £142m move to Barcelona did not adversely affect Liverpool's form, but nobody knew that at the time.

Klopp knows keeping Liverpool's best players is hugely important, which is why he was so pleased external-link when Sadio Mane signed a new long-term contract a few weeks ago.

He saw that as a sign that Liverpool's top players no longer believe they have to leave the club to win trophies, but there is more to it than that.

Mane and the rest of the front three are seen as some of the biggest stars in that team, but they seem settled full-stop.

They seem to love the way their side plays, they like their manager and they have a great affinity with the fans.

For players to be happy, their families have to be happy too and Klopp will have played a big part in that. He is the sort of manager who sees his squad as people, not just players, and it is paying off.

Reds are adaptable - and ready for Man Utd

Before the Napoli game, people were saying how another European night under the Anfield lights would see Liverpool through to the last 16, but that was not the significant factor in the end.

Don't get me wrong, the atmosphere was good but it was not the same as it was during the knock-out stages in Europe last season, or even the opening game in Group C against Paris St-Germain in September.

Anfield did not feel as much of a gladiatorial arena as it has in the past, but Liverpool did not need to be lifted by their fans in order to get through.

Instead, they managed the occasion. It was not an outstanding performance, but they were still pretty comfortable.

The Reds were not as clinical as they have been in front of goal either, but even with the chances they missed I almost always felt as if they were in control of the tie.

It was a really professional performance that is becoming the hallmark of this Liverpool side, rather than the devastating attacking football that took them to last year's final - although that is what they are being compared to.

Their success so far is down to the whole team, not just any one department. Any other season, and people would be raving about what Klopp has done, but at the moment all I am hearing is that the front three are not really clicking.

As I keep saying in response, if that is the case and yet they are still unbeaten and are top of the Premier League, then no-one else stands a chance when they do find some form in front of goal.

The Champions League does not resume until February but the tests are going to keep coming for Liverpool, starting with the visit of Manchester United on Sunday.

Liverpool were afraid of us - Mourinho

It is the sort of game that United boss Jose Mourinho relishes and you know he would love to take centre stage this weekend.

He will not be bothered about open, attacking football - he will come to stop Liverpool and, as he has shown in the past, he is very good at it.

But this Liverpool team can adapt when they have to, for example when they drew 0-0 with Manchester City in October.

That was an extremely cagey game where both sides waited to see who would blink first, and it would not surprise me if things turned out the same way this weekend too.

I would still be confident Liverpool could create enough chances to win, though. As we have seen so many times this season, they do not have to blow teams away in order to get a positive result.

Mark Lawrenson was speaking to BBC Sport's Chris Bevan.