I am absolutely convinced that our problem is mainly mental/psychological. In our times, some things appear to be what they aren't.
Under Tata, we played one of the best half of the decade against Atletico Madrid in the CL return leg but it was one of the worst season of the last decade. That's just my personal opinion, but I still believe that we were potentially the best team in the world, it's just that mentally ( and maybe physically) we weren't there.
Under Enrique, Barca is a lot more versatile and can beat any team on their A game, but I noticed that there were phases, especially since the Arsenal game, were they massively dropped in terms of quality for several minutes. We clearly lack consistency this season.It's a long way to the top but it's even harder to stay at that level. Please correct me if I am wrong under Guardiola, we played football at a very very high level for 4 successive seasons. And every season, Guardiola made adjustmensts. Enrique made minor adjustments this season.The transfer ban and the injuries were actually a blessing in disguise and it helped us to discover new weapons we weren't even aware we had. It helped us improve massively to the extent that we demolished Real in their own backyard without Messi.
Unless someone is part of the Barca staff, I don't think that anyone here really knows why we are less consistent. So I'll just make a guess :
From Guardiola to Tata, we had a very solid construct in the midfield. We were physically inferior, but technically so superior that we forced the opposite team to defend and we dominated them.
With Suarez and Rakitic joining the club Enrique partially broke that construct and developed other aspects of Barca's game.We had to pass through an awakward transition from August 2014 till october 2014 where we managed to win without impressing. But Enrique had very clear ideas, he made the right adjustments and it finished to pay off : Barca played amazing football from February 2015 till the end of the season.
One of the keys to success under Enrique's system, and that's just my guess, is intensity. With all the games we played, it's safe to assume that we are now witnessing a diminished Barcelona. With Messi being out of form/gassed and Arda clearly not being able to bring what a 35 years old Xavi could bring, I am afraid that without our gala XI we lose the "construct" we had in the midfield. Actually, without Iniesta on the pitch, we are not the Barcelona of 2009-2015 anymore. I would argue that Enrique doesn't have the necessary players to further make adjustements. Rakitic is very good but he can't control the pace of the game nor can he overcome opposite pressing in thight areas of the field. Suarez is monstruous and the best number 9 in the world but he can't contribute to our possession game when needed. Neymar has massively improved his passing but he clearly cannot contribute to our build-up game as Messi, usually, does. When you have Rakitic, Suarez and Neymar instead of Xavi,Pedro and Villa you are forced to play a more direct and intense game. But if you#re not on your A game you start to become more clumsy, and when Messi is out of shape ( mentally and physically) you start to lose the construct you had in the midfield. The rest of this season will be very interesting to see how we will evolve.
Anyway this game will be the moment of truth for this season. Iniesta's leadership and Messi's mindset will be key.