Maybe it's just an issue of preference, but I've always been of the view that centre backs with pace are more important than central midfielders with pace; for me, it's more important that the latter have great mobility and stamina. Reading various threads, a lot of posters are getting pace, mobility and stamina confused.
As a centre back, you have to be an exceptional reader of the game to get by without pace - see Vidic at his best. Problem is, his injuries have reduced his mobility, and he's a shell of the player he was.
Quick centre backs are vital tactically - they allow the entire team to play higher and kill the space for opponents to play - too often, United retreat because of a lack of speed at the back, and it makes it easy for our opponents to play. It's an area we need to look at in the summer, amongst others!
As others have said, United have coped without much pace in central midfield over the years. More important is positioning and ball retention.