Some rather harsh criticism being levied against Di María, IMO. The way it appeared was - Di María wanted to go to Paris Saint-Germain, yes. But United wasn't a step down as in if we had gotten our act together, we're as attractive a club to play for as them, if not moreso - given the relative gulf in historic stature of the duo. If say Fergie (or any quality man manager - Ancelotti to name one) was our manager instead of Louis, I have no doubt in my mind that he would've assimilated Di María into the squad, and made him feel as one of our own (Carlo did exactly that in Di María's last season at Real where he was incredibly unsettled to start with, but turned it around to become arguably their second most important player in La Decima in the midfield alongside Modrić and Alonso).
Lots of Iberian and South American players have a tough time adapting, but the key lies with how the manager handles that situation on an individual basis. De Gea has spoken about regretting the the decision to join United too (through the early weeks and months), but Fergie put an arm around the lad, his form improved - and now he seems relatively settled. With Di María, you get the sense that he didn't envisage a chance of success at United (he's been vindicated on that point, for now) and feel appreciated by the manager - who played him up and down in an unsettled starting XI, and likely didn't give him the confidence and backing he needed when things started to go tits up, and didn't devise a system to help him, somewhat.
Yes, he's a professional, and he should get over it - but there are different shades to each player, and when you have a manager like Louis who treats everyone in a monochrome fashion (apart from his select pets, that is), there's bound to be a fallout. Di María was a bit gutless yes, and even when he was given some individual freedom - he didn't really shine in a way that would justify the price-tag; but I'd allocate a nigh equal amount of blame with Louis, who seems really dense on an emotional level, despite harping about 'human qualities', and 'democracy', and other tripe - and probably burnt his bridges with the player, instead of trying to find common ground (bit similar to the Hernández episode).