Signing Di Maria would leave more questions than answers whatever system you play.
If you switch to a 4-3-3 then Mata and 1 of either Rooney or RvP are either being left out or played out of position. And there's no chance of you playing Di Maria as a wing back in a 3-5-2. He'd be replacing one of Mata, Rooney or RvP and you'll play with Di Maria behind a front 2 or you'll play him + Mata behind a single striker.
Utd's biggest problem is something I've said 100 times about Liverpool over the last few years, different managers with different ideas signing different players. Utd have got Rooney and RvP so they have to play them, Moyes then spends £35m+ on Mata so they have to play him and Van Gaal's left with 2 other managers players trying to fit them into his system.
Yeah, I'm not sure what we're going to do. We'd be better off selling more of these players and starting with a clean slate where possible. I think the 3-5-2 fits our players well in theory, but it means we really have to get more players in for central midfield and another defender or two. Evans is our only experienced defender and he seems to be getting more and more injury prone.
Di Maria is very much a winger though, or he can play as part of a midfield 3 (not really behind the striker, he was more of a box to box player for RM last season).
Spending £50m on Di Maria to play him behind the striker would be sheer insanity.