Because he plays in that position doesn't mean he's a CB, from the moment he thinks and acts as a midfielder he won't be a CB. It's already mentioned many times that he lacks the height which is a very important aspect for a defender and especially for a team like Barcelona at least it's defenders should be tall. I don't think he has the sense of positioning and the right understanding of a situation to stop it, that's why he always likes to go first. But that can be dangerous at times, see his mistake against Milan see his wrong positions at Higuain's goal in the Super Cup in Bernabeu. I mean he's not bad at all playing against normal - small sides but he's not the right one to play big games. He's been very good this season but that comes from an image before playing a big game like this, now he's tested in a big game you can see that he can't compete on that kind of games.