His ability on the ball, his ability not to dive into tackles, his positioning is actually pretty good for proper defending but he's not much good on offside traps. He's a good leader and is often able to turn a team performance around.Again, for what specifically is he rated highly for? He can't defend, his positioning is awful, his decision making is appalling and he's a cheat to top it off. His passing is very good and he's shown he can excel playing in front of the back four on rare occasions, but he's a shocking defender.
He is certainly prone to errors, but that's an acceptable trade IMO. If he was always at his best and didn't make errors then he'd be one of the defenders a top club holds and doesn't let go of and he'd never have ended up at Chelsea.