Problem is that his head seems to go when he decides he's being fouled and he starts playing for the ref. Trying to get whistles and feigning injuries and doing all sorts of things that a CB really can't be doing. When he just goes about his business no nonsense he's quite a good player but when he gets frantic and starts trying to do too much he creates such havoc in his own back line that it becomes a huge problem. The best way I can describe it would be to use the gambling analogy that he goes "on tilt" in the middle of a match and there's just no bringing him back. He is awfully comfortable carrying the ball forward and can play a forward pass. He's not strong in the air (lost the duel with Zlatan which could have been called a foul but who knows what is and isn't a foul on those any more). But losing aerial duels to tall players will always happen, he's got to stay composed and within himself and stay disciplined and he'd be just fine.