Fabregas wouldn't get into the first-choice Barcelona XI.
Xavi, Iniesta, Busquets, Pedro and Messi (arguably considered a striker now) leave no room for him, and fit the system completely. The only way Cesc would be able to start would be if one of these players was injured, or in a couple of years when Xavi begins the inevitable decline.
Some might argue that Fabregas would get into the team ahead of Sergio Busquets; I disagree, because having watched the majority of the Barcelona games in the Champions League over the past couple of seasons (with him starting the vast majority), I've begun to notice not only does he offer a holding role within the midfield, but his ability to spread the ball across the field is also impressive.