Why Lionel Messi is Pep Guardiola and Man City's biggest problem - Alan Shearer

Barcelona and Lionel Messi are the biggest reason I don't think Manchester City will win the quadruple.

It would be a great game, but any team with Messi playing for them would be the favourites.

City, of course, have to get there first. They have not faced anyone of note yet in Europe, but if they continue their progress then that is where they will probably meet the toughest opponents of their entire season.

They should get past Tottenham in the quarter-finals, although Tuesday's first leg at Spurs' new stadium will be a big occasion and it is going to be tough for them.

Get through that and it is either Juventus or Ajax in the semi-finals. Again, there is no easy option there, so it is definitely going to be the hardest competition for them to win.

I believe they could win the other three trophies but, on top of everything else, the Champions League could be too much for them.

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'Title race will go to the last weekend'

The fact that, with a maximum of 12 games left, City can look at their remaining fixtures and know they are so close to a clean sweep is an amazing achievement in itself.

City already have the League Cup in the bag and I think they will win the FA Cup.

Watford frustrated them for long periods at Etihad Stadium in March and City only made the breakthrough thanks to Raheem Sterling's controversial opening goal, which was initially ruled out for offside before being given. Playing them in the final will not be easy, but again it is a game where their quality and mentality should see them through.

The Premier League is a lot harder to call, but it is in their own hands - if Guardiola's side win their six remaining league games they are champions.

I don't think they will win all six, but we are talking about a team who have won 22 of their past 23 matches in all competitions so it would not surprise me if they did.

There are some tricky games in there for City though, starting at Crystal Palace next weekend.

*Denotes Champions League quarter-final

Palace have already beaten them this season, before Christmas when Andros Townsend scored a wonder goal at the Etihad, and the Eagles also missed a late penalty against them last season that would have ended City's unbeaten start.

Roy Hodgson's side have been much better away from home than at Selhurst Park this season but they are definitely going to be dangerous opponents.

The obvious game that stands out as being difficult is their trip to Old Trafford later in April but the picture at the top might have changed by then.

I don't expect Liverpool to win every game either, and United away might not be a fixture City have to win to maintain their advantage.

However it pans out, there are going to be more nerve-wracking moments for both teams, and at some point one of them will blink first and drop some points.

But I still see the title race going right to the final day of the season, when Liverpool host Wolves and City are at Brighton.

That is great if you are a neutral, and able to sit and watch what is happening in both games, but having been involved in a scenario like that myself - when my Blackburn side won the title in 1995 - it is not such an enjoyable experience for the teams and players directly involved - at least until the outcome has been decided in your favour.

Aguero absence a blow but De Bruyne return a big boost

To have a chance of succeeding on all four fronts, City ideally need everyone fit and in their best form.

Unfortunately for them, that does not look like happening anytime soon, because people keep getting knocks.

City can cope with some of those setbacks because they have the strongest squad in the country, which is one of the reasons they have reached this stage of the season in the position they are in.

But other absences are harder to handle. For example, they will always miss Sergio Aguero if he is injured, because he is world class. Gabriel Jesus is a very good player but he is not Aguero, who is the best striker in the Premier League.

I am sure Aguero will play against Spurs on Tuesday if he can but if there is any doubt they should leave him out. Nothing will be decided that night and there is so much at stake for the rest of the season.

It is a blow for City that Aguero has picked up a knock at such an important time, but on the other hand it is a big positive that Kevin de Bruyne is back in action.

De Bruyne has struggled badly with injuries since the summer and has not reached the standards he set last season. He has completed 90 minutes only three times so far in 2018-19.

He did not shine against Brighton, but still produced the game-changing pass that ultimately sent City through, with one of those whipped crosses he is so good at.

They will need more of those sort of moments from him in the next few weeks, because they could make the crucial difference.

City do not have to sparkle every time

I don't think the fact City were below their best at Wembley will concern Guardiola too much, because the main thing was getting through the tie.

If City go on to win the FA Cup, then that performance will be forgotten.

The fact it was a poor game, and not much of a spectacle, is not their fault. It was more down to Brighton not being good enough to cause them any problems, even against a City team that played poorly.

In their past three matches, against Fulham, Cardiff and now the Seagulls too, City have scored early on and then coasted. They have not really been tested.

From what I have seen they have been in cruise control, and have never really had to get out of second gear. That is likely to change now, because the teams they face from now will have more quality.

But to get over the line and add more trophies, City will not necessarily have to be at their sparkling best every time. They will undoubtedly have to grind some of those games out the way they did against Brighton.

They will need cool heads as well. Saturday's semi-final would have been very different if Kyle Walker had been sent off after clashing with Alireza Jahanbakhsh in the first half, and he was very fortunate not to get a red card.

Luck will play a part too, but that will be the same for every team trying to win any of the three competitions that are still to be decided. You don't win anything without having it on your side.

Alan Shearer was speaking to BBC Sport's Chris Bevan.