Which is...?
Seriously, it's quite unclear what his role is going to be. I do not think he is a defensive player, he doesn't have the right instincts and awareness for it. He can be a spoiler, yes, one who is on the pitch to "get stuck in" and foul people and to be generally annoying. But you should need far more than that at Manchester United.
Also, it seems that all that talk about being "PL proven" was the rubbish I always suspected it to be. It amounts to a big fat amount of nothing when signing for United.
On paper, Fellaini can partner any of our other CMs comfortably.
With Carrick he can play a box-to-box role pretty much along the lines of what Fletcher used to do, but with more physical presence and aerial ability. This would be with Carrick playing his prefered role of screening the back four.
With Cleverley, Fellaini can drop a bit deeper and play more of a "destroyer" role which frees up Cleverley to play HIS natural game.
Alternately he provides good balance in a 3-man midfield, either as the anchorman with two more creative players (eg Kagawa and Cleverley, or even Rooney) in front of him, or he can be one of the more advanced two with Carrick or Jones holding.
...do we have any other CMs worth mentioning?
Basically I think Fellaini has a lot more in his locker than he has shown so far, and I gave my reasons in my previous post. Again though, he hasnt had any proper preseason or time to train with the squad prior to joining, which for a midfielder/defender I think is crucial, and he hasnt been played constantly by Moyes either*.
*Im not suggesting that he should play every game, but when we signed Berbatov, Fergie started him ever game to play his way into form. You can argue whether it worked or not but Dimitar got a lot of playing time. Unfortuantely due to the fixtures we have had, Moyes hasnt been able to afford Fellaini this luxury - thus he is still very much settling in.