I am not sure if new CM would need to be very mobile, its more a question of how Fellaini would be utilized. With Hull the team seemed to be changing shape when in possession and Fellaini would burst forward. He's certainly a great target to have up there for the quick counter - his chest control is literally best I've seen. So the other CM would need to be more conservative. Mainly they just need to have 'look for a forward pass first' mentality and be able to pull it off - Giggs has no legs but when he showed up, started moving the ball forward and switching the play, it finally started to look like organized football. Verratti would work great here.
But if Fellaini were to sit deeper, then its a different story.
My 0.02$
But if Fellaini were to sit deeper, then its a different story.
My 0.02$