Di maria was easily Man United's most talented player when he was there. He was just moved about by Van Gaal, suffered a robbery. He never wanted to be there in the first place - he was forced out of Real Madrid because the signed Bale the previous summer and didn't wanna keep compromising by playing Di Maria in a quasi attacking midfield role. I still think Real got rid of him way too early but they'll argue they went on to win three CLs so splitting hairs.
I said when he signed for Man U that he'd not do well for them. Mainly because he never wanted to be there, ever.