Regardless of results for City, which are better(though I think a much better team) there has been a massive change in style and tactics under Guardiola. Chelsea look very different with almost the same players under Conte, a 3-4-3 for one thing today. Under Mourinho Utd's offence still looks clueless, he hasn't improved the form of any players, he hasn't changed the feel of the defence, the mentality of the team.
While City and Chelsea haven't come close to finished their journey under their new managers, the direction and change is clear, with Mourinho there just looks to be no plan, no idea, nothing they are moving towards, no particular style being put on the team. Just when it goes wrong, put everyone deep and try not to lose, which isn't even working well.
I honestly think the only thing Mourinho has ever done is be a motivational coach to get the best out of a group of players over a 1-2 year time frame before they burn out. I don't think he has ever created a new or better team, I don't think he has a brilliant system or brilliant tactics, he just persuades players to run themselves into the ground on the style they had before he joined... except getting worse and stale over time. That blueprint can't possibly work on a team that had decent defensive/grind a result mentality under LVG but had no top tactics, style or attack to get the best out of mentally. They need a manager to build something at Utd and Mourinho isn't that guy.
I think Enrique and Simeone will be looking for a new challenge within a year or two and that is Utd's best chance for real change and building a team to win a CL again. Simeone seemingly had his contract length reduced with Atletico as well, so he seems to be off and seems to have a destination, not entirely sure who would obviously be moving on soon... Wenger?