This season, Barcelona have started to look more vulnerable than usual.

After they thumped both Celtic and Manchester City at Camp Nou in the earlier parts of this season, the Catalans sit top of group C, they should really qualify as group winners. But defeat to Manchester City has left Pep Guardiola’s current side within striking distance of his former one.

There’s no suggestion that this is a crisis, nor is there anything to suggest that they won’t beat Celtic and Borussia Monchengladbach and finish top of their group - nothing beyond the cliched fact that anything can happen in a game of football.

Yet the veneer of invincibility does seem to be rubbing off. At the start of the season, I wrote that the pressure of keeping up the pace could be too much for Barcelona this season. But the truth is, if there is a dropping-off in their form and ability this season, it’s much more nuanced than a complete collapse.

The treble win the season before last was followed up with a double last season. You probably don’t have to look beyond the team sheet to conclude that this is one of the best football teams ever assembled, but their trophy haul certainly backs up that theory. It was only their near-implosion towards the end of the season that cost them another shot at defending their Champions League crown, and made their defence of the league title look closer than it should have been.

But that implosion was telling: it was so unexpected, so sudden, and although they put it right in the end, it shows the house of cards they have built. They rely fairly heavily on a front three who play every week and also have to travel to South America during international ‘breaks’. Every other team in history would rely on a front three of Lionel Messi, Luis Suarez and Neymar: the reliance is fair enough, but it creates a brutal schedule for their best players.

That’s the physical strain, but there’s also a mental and emotional pressure - recent court cases may be taking their toll, but it’s probably even more likely that keeping up the sort of form that sees you win doubles and trebles every season is just impossible. That’s why they’re so rare, though they come along at Barcelona more often than they do at most other clubs. But look at Manchester City: their recent history shows they win the league then implode the next season. That’s not because they’re a bad team, it’s because the mental pressure of sustaining that level is so great.

In the weeks after international breaks, Barcelona are vulnerable - MSN are vulnerable. They limped to a 0-0 draw with Malaga at the weekend. That’s ok, draws happen, but Luis Enrique’s side sit in a rather precarious position aside from that.

If they are struggling under mental pressure as well as physical strain, Celtic Park on a fevered European night is the last place they’ll want to visit, especially after the high-pressing masterclass Brendan Rodgers’ side gave Manchester City for an hour in late September.

And then there’s another, very different, pressing question for Luis Enrique. If they do lose to Celtic, that puts Manchester City in a commanding position in the group if they beat Gladbach this week. But if Barcelona don’t rest players, they could very quickly find out that there are worse things than finishing second in the Champions League group stages.

Imagine the scene: their best players are tired, just back from South America. After a draw at home to Malaga, (let’s say, though it’s anything but a given,) they unleash a backlash against Celtic, but then they have to travel to Anoeta to face Real Sociedad before a midweek cup fixture and then a home game against Real Madrid the next weekend.

So that includes a trip to Celtic Park - a trip that has, historically, been tough for Barcelona - and one to Anoeta - home of one of the most shocking stats in world football. Barcelona haven’t won at Real Sociedad’s ground since 2007. That’s before Pep Guardiola took charge. That’s when we still thought Barcelona’s terrible triad was Deco, Samuel Eto’o and Ronaldinho, not Messi, Suarez and Neymar.

This is an important period for Barcelona. Starting it off form will pile pressure onto their shoulders before they go into the games against Reals Sociedad and Madrid. But trying to put right their draw last weekend by invading Celtic Park with guns blazing may only leave themselves open to even more tiredness.

Their squad is better now than it was last season when they imploded, but that doesn’t make this any easier: you can’t really improve on the players they’ve had over the past few years. And rotation will be key.

There should be no doubt that Barcelona are good enough to overcome Celtic and Real Sociedad before a showdown with Real Madrid, but for the first time in years Barcelona look human, they look tired, and they look beatable. And that should be worrying for Luis Enrique. His team face some huge matches over the next few days, and they come into those games having drawn with Malaga at the weekend.

They could find themselves as far as 10 points behind Real Madrid by the time El Clasico is over. But if they weather the storm, they can find themselves all but guaranteed top spot in their Champions League group and right back to Real Madrid’s pace at the top of La Liga, too. The next two weeks really are that important.

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