I'm at the point now where I feel like it must be the fundamental club culture. For almost a decade now, across five different managers and basically three different squads, things have been mostly the same. It doesn't seem realistic that the blame can be tied to either the players or the manager. At this point, of course, the players have been completely soured and lost faith in the whole thing, so I'm not even very surprised that they're being "toxic." If everything was total chaos and non-stop failure at my workplace, I don't think I'd be going to work with enthusiasm and determination. Manchester United F.C. has become a toxic workplace environment, and nobody can thrive under that.
A lot can contribute to this. I've got experience analyzing and untangling TWE (Toxic Workplace Environments), and it's almost never as simple as "the staff are cnuts" or "the boss is incompetent." It'll be something that permeates the whole club. I'll point to such things as the fact that until very recently, regular staff at the club weren't paid a living wage. If you worked as, say, a cook or maintenance worker or whatever at MUFC, it paid less than a job at fecking McDonald's. It was expected that you would see it as a privilege to work for the club and do it for a pittance. And you wouldn't necessarily think that the feelings of the cafeteria staff has much of an impact on player performances, but these are the people they interact with day to day. These are the faces they see when they go to work. They're in that environment, and if the workplace environment is full of people who don't feel very good about it, it affects everything.
Contrast this to reports of City (and probably many other top clubs) who go out of their way to foster positivity at all levels of the club. If you play for a club where everyone loves to be, from the manager and players down to the cafeteria staff and kitmen, there won't be this cloud of negativity surrounding your job. And nobody is immune to the effects such a thing has on the subconscious. These players, who are often very young, are very much affected by the general mood and atmosphere of their workplace. These are young men, often in their early twenties, who are extremely susceptible to emotional influences, for much the same reason that there's a home team advantage because your fans are cheering you on while you play.
We all know that things have been a circus at the club for years. We were the last PL club to start paying a living wage, which I think we started doing just last year. I doubt the everyday workers all greeted the players with smiles and good vibes in the morning. The owners are parasitic businessmen who hijacked the club against its will and installed their Wall Street cronies in positions throughout the hierarchy, and leech money out of the club, money that the players are earning for them. That must surely affect the pride and joy of playing for United.