With the addition of Kagawa, SAF will be playing United in a 4-2-3-1, which seems to be what most big teams are playing, especially in Champions League. Sahin would be playing allongside Carrick, with Kagawa in AM position. Three midfielders are neccessary against the big boys, as we saw last season.
I'm sure now you're going to say, "what about Cleverley?" IMHO, Cleverley still has some learning to do and should be played against mid table-to-weaker P.L teams and stronger FA Cup/Carling Cup opponents, in a more attacking role, with Scholes deeper playmaking, helped by Tunnicliffe/Powell/Petrucci. Remember, SAF likes to rotate his squad, likes to Bllood younger players in Cup competitions and for those players to gain experience from the veterans, especially Scholes.
This gives all the younger players their shot at first team action, and still gives us the strongest line-up for the Champions League and strong P.L. matches as well. I did not include Anderson as he is too inconsistant and can certainly be used off the bench until ready.
Does that answer your question?