Not all successful coaches can achieve good results in a short time, and I think the problem with Manchester United is not giving the coaches enough time. Look at how patient Liverpool was with Klopp. I am afraid with the current trend at United, even the likes of Zidane or Pochettino may just end up getting sacked.
I think this is very inappropriate for the situation. when you say Manchester United didn't give the coach enough time then what happened to Ole? he coached Manchester United for more than 3 seasons and although in the early seasons it was quite good then this season he is even quite hated.
I don't think it's a matter of time, maybe on the one hand time is quite influential but on the other hand when it's been given a long time but still can't show good performance what is it for
Not giving enough time? Ole had like 5 years or something at United and with the amount of money he got, dude never became champions. Klopp had around the same level of money and dude managed to turn that club into a premier league winner and a UCL winner (plus a finals on top of that) in the same time period. I am sorry but United is the last team to say they do not give enough time, they had the greatest coach of all time for like 20+ years, dude became Sir after that.
So, I really doubt you could say United didn't give Ole enough time. If you are looking at their current one, that was just temporary anyway, it wasn't really to keep him, they are not giving him a chance from the start, they just wanted to handle the team until the season is over and that's it.