The thing about Busquets..his skill set is quite unique and what he brings to the team is quite specialised. He isn't quick, doesn't provide much penetration offensively (of late at least) and obviously isn't a physical destroyer. That's why, when he's bad, he's REALLY bad; because if he messes up things like getting out of a tight press and gives the ball away being the last midfielder, then he really can't make up in other areas of the game.
I hate to admit it, but I get the feeling it might be complacency? He's just been an automatic pick for so long...it happens to most people.