Think the United players are now playing in fear of Mourinho. When Mourinho was at Chelsea he was a great motivator and truely led them to glory. Did the same at Inter but he has lost the plot at United.
He isn't creating a siege mentality, he has created an atmosphere of mistrust and fear. His constant complaining over transfer funds are a pre-emptive excuse for not winning the league or dominating. His ego is getting in the way of the team. The player will choke in such an atmosphere. The fans are mostly against him because he doesn't entertain, winning with style is not important to him.
No matter how much money is spent now will not repair the damage Mourinho has done to the overall atmosphere at the club. So still a force but a watered down version of past glories.