At the start of April, Barcelona hosted Real Madrid at the Camp Nou. Barca were champions, they were on course for another league title, and it looked like no one could stop them in Europe either.When Gerard Pique put them ahead in the second half, it looked like their lead at the top of the table would be impossible to beat. It looked like Barcelona would be impossible to beat. But fast forward two weeks and they found themselves level on points with Atletico Madrid and only a point ahead of Real in the league table. In the meantime, they’d managed to lose to Atleti in the Champions League, too.It was a collapse of epic proportions, not least because it was so unexpected. The team of Lionel Messi, Luis Suarez and Neymar looked unstoppable, then suddenly looked beatable.BusquetsIn La Liga, it was four winless games in a row: a draw and then three consecutive defeats. That was the wobble, but it was emphatically ended by the final five league games. Five games, five victories, and an average of almost five goals-per-game. 24 goals in five games showed that Barca were back, baby.But it was the final defeat of the poor run that showed a worrying weakness in Barcelona’s squad.It wasn’t the fact that they lost the game, and looking back on it, it wasn’t a game that mattered in the end. But what was interesting was the fact that although Luis Enrique’s side lost that game, he didn’t make a single substitution in order to change the flow and make a breakthrough.That was, mostly, because he couldn’t. The Barcelona bench that evening consisted of Marc-Andre ter Stegen (the sub goalkeeper) Adriano, Dani Alves, Douglas, Marc Bartra and Aleix Vidal (five defenders) and Munir. None of those players could have changed a game in a way that the magical MSN couldn’t.The bench that night was possibly the most worrying part of Barcelona’s entire season. They won the league and the Copa del Rey. And yes, they didn’t win the Champions League, but no one has ever retained that. And besides, two trebles in a row is obscene. It was a good season apart from the blip. But long-term, that bench was alarming. The lack of depth, the lack of options, and the fact that so late into the season, after so many games, Barcelona couldn’t call on any attacking alternatives to freshen things up.But compare and contrast with this season:

Although they lost to Alaves, Barcelona’s bench that night was arguably the best ever assembled in a competitive football match. And although it was clearly a second string side that lost that game, the fact that they can field a second choice team inclusive of the likes of Javier Mascherano, Arda Turan and Paco Alcacer shows frightening strength in depth.

The players signed in the summer weren’t added to make the starting lineup better. They were added to make the team better. And on the evidence of the Valencia game, the squad certainly needed to be better.

Barcelona have done squad shake-ups quietly over the past few seasons. Lots of money has been spent, of course, but you always get the feeling that Barcelona are just the same Barca who’ve been tearing the world apart since Pep Guardiola took over in 2008, with a few notable additions. They beat Celtic 7-0 just after that Alaves game, but the last time Barcelona lost to Celtic was in 2012: just four of the starting lineup that night still play for the club. But they still feel like the same Barcelona team.

River Plate v FC Barcelona - FIFA Club World Cup Final

Yet such unfettered success is beginning to look like a house of cards that could come tumbling down at any point. For one thing, replacing players like Victor Valdes and Xavi is never easy and can fail spectacularly. At some point, one of those replacements just won’t be as good - that’s just the law of averages, but that’s also where the whole house of cards idea gets its bite. Can Denis Suarez, Andre Gomes or Paco Alcacer replace Lionel Messi, Neymar or Andres Iniesta? Whether that’s long-term, or short-term due to injuries or suspensions?

And then there are the off-the-field problems that the club and its players are facing. Barcelona are facing yet more scrutiny over the transfer of Neymar in a story that just won’t go away. The rising salaries of Messi and Neymar is another potential spot of trouble on the horizon.

And just how much have tax cases and suspended prison sentences taken their toll on Messi and Mascherano?

And then there’s the obvious worry: can you really expect a team like Barcelona to be at the very top of the game for quite so long? They won the treble the season before last, they looked like they could very well win it again last year. This year, they look less unstoppable, but still very good. Yet surely there comes a point when they fail. Surely there comes a point when tiredness creeps in, when travelling to South America for international duty every month finally takes its toll on the MSN axis of pure footballing power?

Barcelona are stronger than they were when they faced Real Madrid in April - a frightening prospect given that at that point, they were on a 39-game unbeaten run. But just because they are stronger on paper doesn’t mean they are less vulnerable. Luis Enrique is building a delicate house of cards at the Camp Nou, the bright side is he’s building it with arguably the best building blocks football has ever seen.