Does any this change the fact he was awful yesterday? It is Manchester United he is playing for. The first 3-4 balls forward he went to play he kicked directly against an opponent. It wasn't good enough and I'm sure the coaches will be telling him that. Sorry if you are offended but he had another nightmare game.
I'm not offended, so we're good, but it seems ridiculous to judge McNair so harshly on a day when almost everyone had a bad day.
And yes, you do have to judge different players differently. I hope would join me in expecting vastly better performances from Wayne Rooney, who has 10 years of experience for club and country, who is on the cusp of breaking the club and country goal scoring record and who earns 300k per week, than Paddy McNair, who has started fewer than 10 matches for United and who is all of 19 years old, who was playing in his first FA Cup match against an opponent playing out of their skins, and who received a hard knock to the head early in the match and who was played out of position due to Rafael's injury.
You have to judge players by where you would reasonably expect them to be at a particular age and the circumstances of the day, not against world class standards.