This would be absolutely horrible. Fellaini is a terrible sitting midfielder. He's slow like Carrick but lacks the positional awareness and the passing ability.
Fellaini never excelled in a 2 man midfield at Everton, which is why its baffling to see Moyes try him in that role now. One would assume he actually knew how he utilized the player at Everton, but it appears not. Fellaini was best at Everton in an advanced midfield role where he worked as the third midfielder, both adding an extra man to help out the midfield duo while also working as a target man. Obviously at United, where we have a quality number 10 option in Rooney, he won't be playing that role, which is a problem because he has to adapt to a new role now if he wants to do a good job for us.
So far hes shown that he is not good enough to play in a midfield two alongside Carrick and dominate, which shouldnt surprise anyone. Technically and ability wise I would put him behind Anderson, who has been tried and failed repeatedly in the same role. He would probably look a lot better if we played a midfield 3, with him as the man ahead of the sitting pair, but then that begs the question how our overall system would look, because thats awfully close to the position Rooney is playing...
I don't really see Fellaini having a natural position in our side, except when Rooney is injured, but even then Kagawa should play the advanced midfield position ahead of him.