Despite a bunch of other valid criticisms the biggest problem is that we are trying to fit certain players into a mould they simply don't suit. I think this is true for both tactics, playing style and positioning but becomes most evident when trying to sit on a 1-goal lead. Players like Smalling and Jones thrive on momentum and confidence, contrast that to someone like John Terry who thrives under the pressure in the pursuit of perfection.
I think that's a good metaphor for the whole club. Fans of clubs like Leicester get energised by watching their plucky boys sitting in. United, from the board, to the ex-players, to the fans in the stadium, to the fans watching at home, get nervous when they do the same. It just doesn't fit the culture of the club.
I have to mention the reaction to Smalling's injury; inexcusably slow considering the opposition only had 10-men as well. The commentator straight away said, drop Matic to CB and have Smalling in the middle. Or Herrera to right back, Lindelof CB, Smalling fluffing around somewhere else.