I've been watching some highlights of Di Maria at Real Madrid, and I think I might be onto something that may explain his performances at United (source video is linked below rather than embedded).
1) ADM usually had 2, maybe 3 attacking players watching ADM ready to make their run towards goal. More killer-pass options means more opportunities for ADM to demonstrate his talent. Our first few games with ADM, we played 4-4-2 with two strikers plus Herrera who is quick and aggressive on the attack, so more passing options up front. The games in which ADM had his relative worst performances were with RVP playing as lone striker and our midfield was fairly defensive.
2) At Real Madrid, ADM had powerful, fast runners to pass to. Benzema and Ronaldo could outsprint the last man and get on the end of the ball. At United, Falcao and Rooney are both quick but Rooney has been playing very deep and Falcao is injured. RVP has a tendency of getting caught offsides or starting his runs too late/not quick enough.
3) The defending in some of these highlights is awful - ADM may be having some difficulty adjusting to the relatively high amount of pressing that occurs in the English game, and the amount of "park the bus" that other teams are playing against us.
#1 and #2 above are relatively easy fixes. Just need to get Falcao and Herrera back in the line-up, and our attacking players need to start getting used to making runs forward in anticipation of a pass from ADM. Right now, it's too predictable - Rooney or Januzaj will run towards ADM to get a sideways pass, leaving only a single attacking player trying to make a run towards goal, which is far too easy to defend against and pass-intercept. Change that so we have multiple players making that same run and we're in business.
#3 above is a bit troubling - it just means more of his killer passes are going to be intercepted, or ADM won't even be able to get off the pass that he wants without that little extra bit of skill, which may be part of the reason why we're seeing some poor passing from ADM.
Still no excuse for his corners though :P