Santon has started 6 games in a row now, Maxwell hasnt been injured, but he lost his spot to this young kid who plays as a solid as a veteran.
Doesnt mean its sure he will play against Man U, but he is the sure starter. If he will benched against Man U, which is somewhat likely even if he played the big derby, then Zanetti is more likely to play there, or Chivu, not Maxwell (liability defensively, but a reccuring thing, but disastrious Atalanta game was the last straw), Mourinho made that clear enough (critized Maxwell).