People have very selective memories where Mata is concerned. Yes he's lost a fair bit of pace, and he was never pacey to begin with. But to say he only played well on Monday because Copenhagen were tired is a bit of a stretch. Before the break, he started most Europa League games and played fine in pretty much all of them, especially the 2nd Club Brugge game where he was excellent. He has the best assist record in the tournament so far, so he must be doing something right.
Against a lot of the faster sides in the PL, he just isn't able to keep up anymore, but I didn't see any evidence to suggest he'd have struggled to keep up against that Copenhagen side. Had he started instead of Rashford (who looks dead on his feet) alongside Fernandes and Pogba (both of whom he's played well alongside in the past) I think his performance would have been fine. Mata has always played well when he's surrounded by good players. It's unfair to judge how he performs alongside the likes of James and Lingard as proof that he's useless overall.
Also I don't see anything particularly bad about saying he played well in his performance thread. The idea that his fans are "hyping him up?" - I don't think anyone is claiming this is some massive career resurgance. We're just merely pointing out he played well, which is allowed last time I checked.