If he plans (and thats a big If) on 4-3-3, then Mata/Kagawa would be used as the creative force.
LVGs opinion of a 4-3-3 is a single pivot with 2 attacking players. So he may plan on playing 2 of Rooney, Kagawa, Mata in these positions with a new signing as the holding player. Problem is he believes Strootman to be the best in that position and he's not fit till the new year.
As for signings, that would mean he is looking at a centre half and potentially 2 wingers.
Fwiw, i dont think he is certain to play 4-3-3.