I'd been gradually doing some writing on how poor Mourinho has been so might as well twist the knife now he is gone.
Reality is Mourinho has been a complete and utter failure. Spent £150m in his first season, had 5/8 of the highest paid players in the league including the top 3 (2 of which he signed himself - Pogba and Ibrahimovic), went on a 22 game unbeaten run only to finish lower than Louis van Gaal and with a lower win % than David Moyes!
His 'treble' was a joke, Southampton were robbed by poor officiating in that League Cup final (surprise surprise) and deserved to win. The run to the final was weak with the exception of one team: Northampton (A), Man City (H), West Ham (H), Hull City in the semis. He also had the easiest run of any team that has made it to a Europa League final since the competition was rebranded. The only thing that prevented him from being sacked season one and consigned to the same scrap heap as Van Gaal was this extremely weak run, while Van Gaal had to face Klopp's Liverpool and presumably would have then been drawn against Dortmund, Villarreal, and Sevilla who had won it the last 2 times in the final. The media love-in meant the BBC weren't going to be manufacturing much pressure at this point. In typical modern Mourinho fashion, no improvement was made in the first year and he blames his predecessor for this with everything being seen as work towards the big second season.
So after a summer of spending about £145m for a new spine of the team in Lindelof, Matic, and Lukaku Man United should have been ready to challenge for the title. They still had the top paid player in the league in Pogba, and Lukaku was on about £250,000 a week, Ibra re-signed pocketing half of that for very little contribution - yet they were already 12 points behind City by Christmas. United had a very good opportunity to open the race back up when they played City at home in early December but Mou had United parking the bus and they were beaten pretty easily (obviously, he blamed the ref). They could have closed the gap to 5 points but it was instead 11 and that was when the race was outright over. This is where the excuses started to come in full flow and he made noises about leaving before signing that new contract in January.
You can argue that nobody was ever going to catch City last year and while that is true the manner of United's title 'challenge' was poor. Their points total would only have been enough to win the league 3 times since the year 2000 (00/01, 10/11, 15/16 on goal difference only). His net spend in the summer of 17/18 was only 4m less than Man City's and he still ended up finishing 19 points behind City, the same number he did in his first season. City did add Laporte in January but then United by that point had Sanchez who had become the league's highest-paid player. Total spendings/net spends do not take into account the crazy amount of money Mou had United throwing at players on wages.
Can't use other team's spending as an excuse when you get knocked out by Bristol City in the League Cup either, or even that limp defeat to Conte's Chelsea. Conte's net spend - around £92m. Mourinho's net spend - around £240m. Not to mention getting dumped out of Europe by Sevilla after parking the bus home and away.
Come round to season 3, his infamous breakdown year and he was whining. The board stopped him from buying a defender like he wanted but, really, can you blame them? Mourinho had spent £60m on two centre-halves, Lindelof and Bailly, and thrown them both aside, complaining that the former especially was rubbish and demolished his confidence before swinging round and targetting the latter as the season progressed. Mourinho's record developing young players is atrocious - he does not do it, he wants the finished article. As the United board became more aware of Mou's waste they wanted a younger defender who could develop and had sell-on value - makes sense. Mou's desperate need for a defender didn't stop him spending £47m on Fred, a midfielder he barely played and looked to have no need for. Then you have the conflicts and attacks on his own players - Shaw, Sanchez, Bailly, Lindelof, Pogba, Lukaku, Martial... etc.
The man is toxic. He can't develop young players, destroys players' confidence and creates disharmony wherever he goes. He has wasted an obscene amount of money on poor transfers and stupid wages. His record at United is a track record of failure only sustained by the weakest of Europa League runs that prevents him being mentioned in the same breath as Moyes and Van Gaal, a level that he belongs in. Good riddance.