I personally believe the traditional 4-4-2 formation suits ManUtd the most. With Rooney playing as one of the center forward, pairing with either RvP or Falcao.
Di Maria is never a center forward. This is not his natural position. Maybe LvG has mistakenly think he is the next CR7. Di Maria's best position is in right wing, with back support from Valencia or Rafael.
I guess LvG has to back up both the defense, as well as the left wing. ManUtd currently lack a natural left winger, after Gissy left. :(