At United though we've always played ball-playing midfielders - even the likes of Keane were very good passers. We don't do destroyers.
Hargreaves obviously would have fit that bill, but then again...
I don't think we would play him as a DM, it would be rather odd. The man has shown he has a great deal of skill both offensively and defensively so limiting him to just playing one way would be negative for both us and him.
I believe we would play him in a 5 man midfield if we were to get him in front of a more accurate passer which Moyes has always had an eye for. Assuming Rooney leaves it would look something like this;
Rafael--------CB------CB--------Evra/Fabio
-------------Carrick/Cleverley------------
--Winger---Fellaini----Kagawa----Winger
--------------------RVP
Moyes enjoys a 5 man midfield(usually 2 CM 1 AM though), narrow wingers and very wide offensive wing-backs.
I think Fellaini and Kagawa would do very well together in a three man midfield. Kagawa would be the number 10 the majority of the times but Fellaini definitely has very good skills taking over the number 10 role when Kagawa drops deep(which happens frequently).
Fellaini is too simple to play as an AM, lacks the positional discipline to sit as a destroyer, has too weak passing to be a deep lying play-maker.
He is very good pseudo CM/AM however and is very good defensively and on set pieces. I think Kagawa is a very good CM/AM as well which suits it.