Man united changed 4 managers at different profiles as you like but all weren't work at all.
Just simply because it weren't a manager problem, all the bosses top up there are the main issue.
They are businessmen, they wanted to push the club marketing value as high as possible, maximizing their shares price.
One of the way of doing so is signing the so called "marketable" players like Ronaldo and Pogba.
Managers have to figure out how to play them, Ronaldo is not happy on the bench, playing is like 90 mins playing with 10 men and he scored a winning goal every fans think he is still one of the best.
If result goes wrong, scapegoat will be the manager, they changed manger, overnight shares price push up again, everything back to the best, the circle will be continued, never ended with this kind of making profit better than winning business strategy.