As has been alluded to, I think him and Carrick being played deep gives the front 4 (Hopefully Kagawa-Rooney-Nani behind RVP) full licence to push forward and attack with abandon without compromising us defensively. I've gone on record about how my biggest issue with our midfield is not so much that it lacks incision (it does, but hopefully the forwards in good form can more than compensate) but that seemingly any opposition player with dribbling ability somewhere between Messi and a potato can just drive unchallenged though the middle 9 times out of 10. Carrick is/was our best defensively minded CM but he's always excelled at reading the game and making interceptions rather than direct challenges (a bit like Rio in that respect) and Clev/Ando just lack defensive prowess at the moment. Fellaini will, I hope, be extremely complementary to Carrick's defending, in the same sense that at their peak Vidic was the blood-and-thunder get a boot or a head to everything type of player, and Rio was the more calm, sophisticated one who swept up whatever was left over.