I see where you're coming from, but he wouldn't fit in as part of the 3. Maybe if he got bucket loads more confidence and resilience about himself, and then improved defensively, he could be part of the 2. There needs to be somebody with pace, quality and directness on the other side though, someone like Welbeck or Nani. Really hope we see something like that on Tuesday with a normal defence. The passing was much improved throughout as well.
The reason it works better is, Rooney isn't capable of "Pass and Move" because of his physique, he can't get into the next position as fast as a Mata or Kagawa can, and turn and take the ball that fast.
Kagawa and Mata are capable of doing it in tandem because they have the same footballing brain and they are singing from the same page, it's been obvious since the start of the system, the reason this is difficult against city is they play a narrow midfield, with wing-backs overlapping, so we need atleast 3 players in the midfield, not 2.
Or we are going to face a problem, so Fellaini Carrick, Fletcher possibly, then with Mata, Rooney, and Kagawa as a narrow 3 to try and penetrate, it's difficult because City are very difficult to play this season, but yes Welbeck is a great option there too, it's trying to fit players that can effect the game, but also pegging players like Touré back because he is the main influence in the city game, Nasri is a player that can be bullied and shut out of a game, Silva is a different story.
If Utd were to play Welbeck against city, his instructions should be clear, "Run at Demichelis".