I don't care as much what a defender is like on the ball, I need them to be able to defend. That involves marking the right man, compressing space, covering team mates, attacking the ball etc.
I've lost count of the number of times this season that Casey has lost his man (or been nowhere near him) and been responsible for conceding a soft goal. He gets less stick from fans because he runs about a lot and makes faces, but his actual defending skills are pretty limited.