Yeah seems there is something complicated happens in Man United, which it began right after Sir Alex retired. Man United management have been trying many ways to solve it by hiring great coach such as van Gaal and Mourinho even gave them huge fund to spend it on transfer windows. For Mourinho, I think the problem is he hasnt found suitable player for his formation yet, so I can say all of his signing on last transfer window is totally useless.
Well it's not all totally useless. Ibrahimovic is generating a lot of income for United in terms of shirt sales and ticket sales.

I have reason to believe that the return of Paul Pogba also has generated a lot of income for them.
Anyway in all seriousness, if Man United does decide to give Mourniho the sack without giving him a real chance to revamp the club, then no good or elite manager would be willing to take the helm at the club. It's really obvious that they need to start from scratch again and accept the fact that the glory days in the Ferguson era is gone.
If we talk about performance on the pitch, then Mou's signing is useless right?

Anw Ibrahimovic shirt sales has been spent to bring Pogba back, so I dont think Man United really got good income from that.
Imo, Man United should beg Sir Alex to get back as the coach at least for one season, so he can choose a potential coach himself as his assistant and after that he can retired again and his assistant could be his successor.
Dude, that man is 74 years old and you want him to crawl back to his old office. It will be fine if he do save MU from the mud, but what if he fail too, that will ruin his managing legacy, people will not see Ferguson as a legend anymore. He is living really well in retire, watching football matches and sometimes criticize or praise some players, but don't ever come out from retirement because even the best can't help Man United at this point.