The problem is, it depends whether you take the short or long-term view
In my opinion, Jose Mourinho is still one of the best managers in world football. It boggles my mind to think that managers like Ole Gunnar Solskjaer and Frank Lampard have top jobs in the PL whilst Jose sits and watches in the Sky Sports studio. Jose probably forgot more about football management yesterday than these chaps will ever know - no disrespect but let's not underestimate Jose's achievements, which are comparable to Clough, SAF and Shankly.
However, there is a serious point to be made here, I'm not a Mourinho fanboy (like some on here) and I accept that Jose made many bad decisions during his final year at the club.
The problem, as I see it, is that IF you are going to appoint a manager like Jose, you have to be ready and able to do everything it takes to get immediate results. That includes indulging his ego and giving him what he wants, when he wants it. If that means spending £300m, it means spending £300m. If it means selling your best player to re-establish authority, it means selling your best player. If it means selling two CBs that manager purchased and buying two more, it means buying two more. There is literally zero point taking on Jose and expecting him to be happy with mediocrity, with a wishy-washy target of a top four finish and blooding the odd young player. Why would (or should) a top manager like Jose put up with that?
Look at the way City have indulged Guardiola. I often see the same criticism levelled at Guardiola - "well....I would love to see how he would get on at Burnley", which in my book is rather irrelevant because the fact is, Guardiola is an expert at getting the best out of elite players. However, City gave Guardiola the platform, they gave him virtually unlimited funds to construct a squad, they never questioned any of his decisions (like bombing out their one 'homegrown' player and captain, Joe Hart), they understood what Guardiola was about and created the conditions he needed to suceed.
You could go into all kinds of detail here but just to summarise, my point is, there's certainly fault to find in the way Jose handled his last 12-months at United, but to use an analogy, it's pointless spending £200K on a brand-new Ferrari and complaining it's a bad car because it won't tow a plough. Likewise, no point buying a top of the range John Deere and claiming it's a bad piece of kit because you're being overtaken on the motorway by 10yr old Fiat's. It's 'horses for courses', and we were the wrong club at the wrong time for Jose, given what we know now in hindsight