I agree with
@Berbasbullet the goal should have stood purely for the fact the ref didn't blow his whistle.
But ESR was wrong to go for goal, usually when keepers are down players don't go for goal because it isn't sporting, your example is a keeper fully at fault as he assumed the ball was dead (it wasn't) and Nani rightfully capitalised on a keeper error.
De Gea had his heel stamped on with studs in the freezing cold, went down assuming the ball was cleared and the opposing team would do the correct thing if they recovered possession or ref to do his job and blow up.
Guarantee if a keeper goes down in a future game the ref will blow up, if he doesn't he's being a bad ref, saying that if it happens in our game, we'd better not be the "bigger team" and put the ball out of play - if it's good for one team its good for all.
Posters arguing that it would have set a precedent are forgetting the fact that this incident should also set a precedent, no keepers are allowed to be injured now, and if they are, a goal can be scored and VAR to assess the validity of the injury purely from video.