We don't have to look for the worst, but if the question is asked then it probably has an answer. Just like you think both O'Shea and Neville are worse. You didn't have to look for worse players.
If I recall correctly you think Bailly is really bad. Maybe you just can't accept that he's a decent defender plagued by fitness issues.
When the United midfield is so exceptionally weak it shouldn't be surprising that the guy with the most games in that midfield isn't rated. Maybe it's wrong, maybe either the midfield is so poor for other reasons, maybe McTominay is good but not a good fit, or maybe the midfield is good without anyone noticing, but that doesn't have to mean that people can't accept the truth or are looking for ways to discredit him.
I think you're wrong about O'Shea, very wrong, but I don't think there's anymore in that than we judging two players differently.