Honestly, he seems like he's one of the best players on the pitch every time he plays. I don't think he needed time on the training ground to understand what Van Gaal wants (apart from when he first came to United probably), but I think he's just not starting every game because of the competition we have for the 3 in the middle. For the holding midfielders, we all know that's not his best position, though he's decent at it. Schweinsteiger and Schneiderlin should be starting there when fit. Mata has the right wing spot his own, so to play, he needs to start as a #10. Performance wise, he is fully deserving of starting every game, but his problem is he needs to compete with Rooney basically. To have herrera in, we need to play either Rooney or Martial on the wing, or bench one of them, and Rooney won't be benched as he's the captain. I'm sure Van Gaal does rate him highly, it's just he values Rooney's presence as captain really highly so doesn't want to drop him.