On the flipside, how did Chelsea still manage to do better than this under interim or short-term managers such as Hiddink, Di Matteo, Grant, Benitez? Why didn’t those managers have their authority questioned by the players during their time there?
Firstly, at Chelsea, the overarching philosophy that runs through the club comes from upstairs. They don't wait for any manager to provide them with one. From Ranieri to Tuchel (Sarri and Lampard are the exceptions, but they both didn't last long), they appoint managers who prioritize good shape and adequate defensive organization. This leads to assembling a squad that, for the most part, is quite homogenous. It's their identity and you can see that their supporters are always ready to get behind their team whenever they assume the role of the underdog in big matches.
Which leads us to the most important part. With Roman's cash, Chelsea's squads are usually good enough to win titles, if a failing manager is replaced midway through the season. Most of the materials are already there, sometimes all they need is a new plan that will bring a breath of fresh air. And, of course, the players know that. So, if they see that the new instructions suit them, even as a short-term solution, they shut up and do their part. No one says no to lifting a trophy or two come the end of the season.
United wait for the manager to do all that and they can't even be consistent when they peak them. Yet, they expect everything to come together somehow by the grace of giving the manager time. Forget about Rangnick for a moment. Ask yourself this: This particular team, what kind of football can they play well? Can they press? Can they play possession football? Can they play out of the back? Then ask yourself: Why did they appoint Rangnick? The umbilical cord between any manager and the club's identity has to be severed. The assessing of the squad has to become an ever going process, regardless of who's at the helm. That's how you create a safety net to fall back on when things don't turn out as planned. Like Chelsea.