I think he'd be one of the best defensive midfielders in the world if he was offered the luxury of playing with 2 technical versatile midfielders ahead of him in a 4-3-3 formation. Unfortunately for Carrick, we insist on playing 4-4-2 with players like Rooney and Welbeck in the space between the targetman and midfield. Not a lot of technique, creativity and overlapping runs there.
I can guarantee that Carrick would have been a massive success for any top team that plays positive attacking football and actually have other good CMs. Carrick has to play with the likes of Cleverley, Fletcher, Fellaini and players who don't belong in a midfield two, like Jones and Kagawa. That's not an easy task.