You do understand the phrase 'coming to a head', don't you? Things don't immediately start rotting. A player doesn't start feeling resentful when he misses a game or two, and another off-form player is preferred. He starts being so when that pattern develops over time. When Bailly sees Maguire rushed back against Leicester, only to make big errors leading to goals, and still get picked ...
By the time you want to get a new manager in, it's too late. You have Martial saying stuff like whether or not a new manager comes in, I want to leave United (as he said almost the moment Rangnick was appointed). What's that to do with the new manager? It's to do with the atmosphere that had built up over the previous regime.
You, among others here, underestimate just what damage Solskjaer did.