Don't want to sound like a douche, but this is expected.
I'd bet anything that his best performances would always come from games where he plays next to Xavi. He's an attacking type of player and he needs to have that midfield control and stability that Xavi brings, so he can be free to do what he does best. When it's Cesc or Iniesta next to him, he's limited and he makes mistakes which are only expectable when you put that kind of a player in Xavi's role. As much as it's nice to see him do well, it sucks to realize that, again, we have many choices for Iniesta's position (Iniesta, Cesc, Thiago), and no good ones for Xavi's. Even after promoting Thiago and bringing Fabregas. I'd honestly rather see Iniesta drop back to Xavi's role when Xavi calls it a day, and let Cesc and Thiago rotate in the attacking position.