None of that explains why Van Gaal has repeatedly played Di Maria and Mata, or Di Maria and Januzaj in the same team. In fact, he's played one of those two pairings in almost every game we've played this season.
Because they haven't both been part of the midfield 3 at the same time. They certainly haven't all three played together.
It's been:
Carrick/Blind
Rooney - Di Maria
Mata
Or:
Carrick/Blind
Januzaj - Rooney
Di Maria
Replace Rooney for Fellaini where necessary.Rooney - Di Maria
Mata
Or:
Carrick/Blind
Januzaj - Rooney
Di Maria
It's always been:
1 DM: Blind or Carrick
1 B2B: Rooney or Fellaini
1 AM/Wide Midfielder: Di Maria or Januzaj
1 #10: Mata, Di Maria, Rooney or Herrera
Herrera was tried as the B2B early on but it seems Van Gaal didn't think of him as complete/physical/defensive enough for that role.
He either battles it out with Di Maria or Mata for the #10, depending on the composition the of 3 behind him or he battles it out with Di Maria or Januzaj for the AM/WM role, both of whom carry the ball forward better and add genuine width and pace to an otherwise narrow formation.