Barca's problem right now is the board - they're unthinkably stupid. People might not think it but Thiago was a very stupid sale. They're struggling right now in midfield because Xavi doesn't have a natural backup and is playing too many games at his age. Thiago would've been perfectly good to play when Xavi needs resting but they haven't been able to do that because they thought it fine to sell him.
The other thing is the team has not at all adjusted to the way teams now defend against them. It was why Guardiola was experimenting so much in his final season and why he was starting to switch to three at the back and play a forward ahead of Messi - Messi gets caged off by four players now and cannot be expected to carry the team on his own. The whole reason they struggled against PSG, Milan and ultimately Bayern in the Champions League last season was that they could not stretch the pitch or create space for Messi and they ended up keeping possession out of caution rather than any actual reason. The team had become so one-dimensional which wasn't at all what Guardiola ever did.
It's also a mess in two defensive areas - they're easily countered against now. Xavi is slow, Fabregas is slow, there's space either side of the centre backs because not only do you have Alves going forward, you also have Alba on the other side (who was a very poorly thought-out signing imo). It's become easier to stop them in possession just remaining narrow and compact and then countering.
The other thing is they've just failed to think about set pieces when signing players. Their tall players have disappeared (again, why sign Alba to replace Abidal?) and whatever system they use to mark at corners, they're always going to be vulnerable.
This is what happens when you lose Guardiola as your manager.
