I posted elsewhere about this and you are so right imho. He single-handled set you back 10 years. First, in one season he managed to turn a footballing winning machine into a very mediocre side. At a stroke, he managed to wipe out the sense of invincibility at Old Trafford that had been built up for decades. Teams no longer turning up expecting a thrashing, but actually thinking "we can win this".
Then by sacking him - which you rightly had to do - he effectively puts all future managers under huge pressure. Gone was the "doing things the proper way - the United way" mantra. United became just like any other club demanding instant success. This was only reinforced further when you (also rightly imho) sacked LVG.
Now you have Mourinho, who even with his CV is obviously already under enormous pressure. If you'd hired Mourinho in the first place, you could have told him he had 3 years to build what he likes with no pressure to win anything. He'd have had you in the top 4 in season 1 (maybe even won it) and winning it in season 2 and beyond.
But after 3 years of underachievement already, Mourinho now doesn't have that luxury. He looks like a man who thinks he has to deliver this season or he's fired.