Anyhow, the big question is who will replace him. Names currently floating around in the news are: Roberto De Zerbi, Thomas Tuchel, Kieran McKenna, Mauricio Pochettino and Thomas Frank.
This is the essence of the problem. Maybe ETH doesn't look like the best of the best, but who is the alternative?
All those listed are good managers, they are rightly mentioned as a possible replacement, but I don't see that any of them has enough capacity for what awaits him in MU. All of them are excellent when there are no great expectations from them and when they are not under the scrutiny of the public.
De Zerbi, had a great last season when he succeeded Graham Potter. No one expected much and not much attention was paid to his work. This allowed him to work relaxed and he produced results.
Pochettino, had some success with Tottenham if you call it success even though you didn't win anything. Then when he had to take the final step forward, he completely failed. Of course, even then no one expected Tottenham to do anything more significant and there were no criticisms when they played badly. Everything fell apart when the criticism became stronger.
Thomas Tuchel is very conflicted, a very similar nature to Mourinho. If he didn't succeed in Bayern, which was much better organized, why would he manage to sort out the mess that is in MU? Also failed in PSG, and the CL title with Chelsea at a time when no one expected anything from them. The following season, everything fell apart when more was expected.
McKenna was already at MU, as an assistant to Ole. Many blamed him for Ole's failures. He is promising, I believe he will be a big name one day, but it is too early for him to take over MU now. He needs to make several successful seasons in a row.
Thomas Frank also fell under the pressure of the previous successful season. It's one thing to lead a team that is content if it's not fighting for survival, and it's quite another thing to lead MU where only wins are satisfying.