Firstly, we're not necessarily talking about players who were good all season, but had one or two poor games, we're talking about players who have had ups and downs, or have one or two flaws in their game that people start to see at the exclusion of everything else. But you've issued the challenge, so I guess I'd better meet it.
Juan Mata gets all sorts of criticism, despite being decent most games.
Phil Jones cops an unreasonable amount of stick on here, yes he's a bit wild, but hardly a bad defender.
The criticism of Lingard is pretty out of control too, just go to any matchday thread from the last couple of months.
Even I'm guilty of giving Fellani a hard time, even when he offers something (I was especially critical of him against Leicester because I thought he slowed us down, but he did offer a different dimension to our attacking play).
Darmian seems to get criticism pretty much every game these days, but he's only put in a couple of poor performances.
The point I'm making is that supporters are melodramatic, especially in matchday threads. More to the point, people tend to take note of individual errors much more than tactical roles (a great example was against Leicester, when Rojo made a few defensive errors and was criticised by everyone, including me, but actually provided a really important dimension to our attack in getting high up the pitch and pinning Schlupp in his own half). Bad games tend to stick in supporters' minds for months, and if they see the same kind of error from the same player a couple of times, then it often becomes an error they 'always' make. Obviously all this is a generalisation, but my point still stands. We, the fans, are not experts, much as we'd like to think so, and I trust the judgement of professionals far more than I trust the judgement of the masses on this forum, myself included.