Pique + Abidal started to be a regular pairing ever since Puyol got injured in mid-January. The scores weren't brilliant, play was often dodgy and it's exactly that time when Pique was having a fuckup after fuckup against Bilbao (if I remember well) and whoever else. Abi did his job for a majority of time as a CB, but Pique was different. He isn't a reliable tool without Puyi and unless he changes that then stability at the back will be a concern since the most important players to your system are getting older (that is Puyi, Xavi, Abi, even Alves but he has more seasons to go than them).
Piqué didn't play that well last season (he didn't play BADLY), whoever his partner was, until Pep made the comments about how he'd only stay on if the players continued to show him the same hunger and drive they have shown in season's past. and that happened in Jan/Feb, and Piqué's form picked up, to the point where he was downright flawless in April and May, again, regardless of opponent.
the most important players to our system are Xavi, Messi, Iniesta and Busquets. and we've taken steps this summer (signing Cesc) to ensure we'll be in a good position to transition away from Xavi's reign. Dani is very important too, but he's supremely fit so there's no worry about him, not for a while yet.
Abidal isn't crucial beyond being a very good defender. we can and will replace him.
Puyol isn't crucial to the system, but he is
fundamentally crucial to our strength of performance. I don't know that we'll be able to replace him at all. Coates could give us the heroic defending, but could he be a unifying force? we have some excellent young defenders coming through La Masia, but will they have the balls that Puyi does? it's very hard to say. we'll definitely lose some bite, some edge, when Puyol retires. I think that is inescapable. we just have to hope that we can sign/promote defenders which enable us to keep Puyi on a limited schedule, thus extending his career and his influence at the club.
Barca does defend with the ball, but then again, when Jan-Feb-Mar period comes by defensive stability becomes more important since not everyone has the fitness (and with the strikes delaying the fixtures which will probably cause more games in midweek) to contain possession at all times.
which is why we've reinforced the midfield so that doesn't happen!
Xavi + Iniesta + Cesc + Thiago means, barring a horrific injury crisis, we should always have a fit and fresh midfield.