For me its like 80% fatigue, which was due to our lack of quality squad players. The only true players who really offer anything for us off the bench is Rafinha and maybe on a good day Vidal, but he hasn't played at all. Arda has just been really disappointing and he was supposed to be someone who could offer us more depth and it's clear he is not clicking here. Remember that we could only make two signings this season so that really limited our ability to gain depth as well.
I think its important however to take this in perspective because granted we hit a bad run of form at a crucial time, this season it felt more like we just didn't have enough gas in us unlike in the past where it was systemic failure. Big difference and the main dent we had was getting knocked out of CL and even still, despite the fact that we didn't play up to standard in the 2nd leg and in parts of the 1st, we didn't get blown out. It wasn't 7-0 like Bayern or anything, it was a very close tie and the same against Real, where we were the better team for a majority of that game as well. The gigs not up, we haven't been found out or exposed, we just slowed down and the games caught up with everyone. With more depth and more options next season, no reason to expect Barca can't win it all again, so its not a crisis or collapse. Particularly if we maintain a domestic double.
Lack of quality players in the squad caused lack of intensity and defensive involvement from all the players, particularly the three forwards.
Real Madrid and Bayern got the likes of Isco, James, Jese, Varane, Ribery/Costa, Gotze on the bench while we have Munir, Rafinha, Bartra, Vermaelen.
My opinion is that we need to sell Vermaelen, Munir, Arda and buy a young world class center back like Marquinhos to compete with Masche and Pique, + a versatile forward like Dybala. We also need a midfielder to rotate with Iniesta and Rakitic, because Arda just doesn't fit. I wouldn't mind Saul or Koke.
Like yes 95% except Saul or Koke. Believe it or not, the player from Atletico who I think would best fit us is Oliver Torres, despite the fact that he hasn't looked the greatest this season. I mainly think that's due to stylistic differences in his play and what Simeone needs from his players and he usually plays on the wing when his best position is through the center.
For me, Marquinhos, Dybala, and either Tielemans, Bazoer, Oliver Torres or Verratti/Gundogan (if we have to sign a big name MF) and that would be a good summer. We kinda need a LB sub for Alba but honestly, Adriano can get it done for at least another season subbing in CdR games.