The problem is that being a hard working player who has minimal impact on a game is good enough for, say, West Brom. It's not good enough for a team who are trying to win the league. You have to be hard working AND have an impact on the game. You can't have situations where the time you are most noticed is when you are walking off the pitch to be replaced by someone better than you.
You look at United sides in the past who won titles, and every player in there is doing a job that is very noticeable. We had players who were in there to be disciplined and hard working, like Park, or Fletcher, but they still had quality, and would have an impact on the game...and they WOULD be disciplined. I know exactly what Fletcher and Park were used for...I don't really have a clue what Lingard is supposed to be doing, most of the time...because what he is doing that is actually useful, is nothing.
The issue I have with Lingard is this:
a) he isn't actually that disciplined. The team often loses shape when in possession because he thinks he's a no10 even when we already have someone else playing there. When we lose possession his side of the pitch is often exposed as he'll have wandered off somewhere to do nothing in particular. This happened repeatedly against Real. This would be annoying regardless of his level of talent. If you're going to drift out of position, it had better be to do something better than get in another player's way.
b) He's not very good at anything, to put it bluntly. He has a good shot on him, he can run fast, but that's it. Again, if you're not as talented as other players, you HAVE to make yourself noticeable or useful in other ways, by making an impact on games or your team's ability to perform in them. Against Real the only notable thing he did in 45 minutes was to get booked fouling someone whilst about 50 yards away from where he was supposed to be on the pitch. This would be fine if it was just a one off, or the odd bad performance, but this is the norm from Lingard. There is very rarely a game where you can say he is even a minor difference maker. Only on the rare occasion where he thumps the ball in from 25 yards...which mainly happens when we're already winning. In the Real game, Rashford came on at half time, played badly, but was a significant improvement, because he is both a threat and intelligent enough to know how to position himself to affect the game...nothing to do with being more talented, which he also is. Just about having the awareness to know how you can make yourself most useful. Generally, average players who make it in good teams, have this awareness...with Lingard half the time I'm not even sure if he's aware what the score is.
Lingard just isn't good enough, basically. He isn't an awful player but he's the sort you'd expect to crop up in the Carling Cup, or when we have our annual injury crisis. He isn't the sort of player you expect to appear 40+ times a season, or as part of a first choice 11 against the likes of City or Real. This is one of the reasons we end up finishing 6th, which in turn is why fans turn their anger upon it.