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But the Old Trafford hierarchy finally served the divorce papers today as the world's biggest club sacked the game's most famous manager.
The decision came with United having laboured to their worst start for 28 years. It has played dull, defence-minded football, with Mourinho cutting an ever-angrier figure after each setback. Monday's 3-1 loss at Liverpool was one humiliation too far.
Having been knocked off their perch by the success of former United manager Alex Ferguson, Liverpool showed just how far back up they have climbed on Sunday as they brushed United aside like the mediocre mid-table team they have become.
With the Liverpool fans singing "Don't sack Mourinho" it was the end of the line for the Portuguese coach after two-and-a-half years - and came 11 months after he extended his contract.
"Manchester United announces that manager Jose Mourinho has left the club with immediate effect," the 20-times English league title winners said in a brief statement.
The club's American owners will have been pleased to see an immediate rise in United's share price, though most fans were more concerned about who would take over one of the most prestigious jobs in world sport.
Tottenham Hotspur's Mauricio Pochettino was installed as favourite straight away and his odds shortened after he gave a less-than-convincing outline of his commitment to Spurs.
"There are a lot of rumours about my position as manager at Tottenham. I cannot answer this type of question," the Argentine told a news conference.
Monday's defeat left United in sixth place trailing 19 points behind Premier League leaders Liverpool and 11 points off the Champions League qualifying places. The 29 goals they have conceded is their worst at this stage of a season for 56 years.
For the current crop of United fans and officials who gorged on success during Ferguson's 26-year reign that is just not acceptable.
Mourinho will point out that after replacing Dutchman Louis van Gaal in May 2016 he won the Europa League and League Cup in his first season, before guiding United to second place and the FA Cup final, where they were beaten by Chelsea, in his second.
But this season has been a relative disaster and, with each passing defeat, he found new ways to blame the players while at the same time reminding his critics of his previous successes at Porto, Chelsea, Inter Milan and Real Madrid.
If he had failed while trying to win with United's customary panache he may have survived a little longer.
But while Manchester City, Liverpool and Spurs have been thrilling old fans and winning new ones with their swashbuckling approach, Mourinho, now 55, has become the arch-proponent of "parking the bus" - a phrase he introduced to English football's lexicon over a decade ago when complaining about teams packing their defences to foil his exciting Chelsea team.
His fallout with French midfielder Paul Pogba summed up his failure.
Good enough to inspire France to win the World Cup this year, Pogba has spent the last two weeks sitting on the bench, having earlier been stripped of the vice-captaincy for daring to suggest the team should be more attacking and play like the Wolverhampton Wanderers side who drew 1-1 at Old Trafford.
Instead, Mourinho has opted for the sturdier qualities of the likes of Nemanja Matic and Marouane Fellaini, while he has been ever-more critical of his players, accusing them of lacking technical expertise, mental fortitude and physical resilience.
The smiling, charming coach who arrived at Chelsea declaring himself "a special one" 14 years ago has long been replaced by a surly, haggard-looking operator, generally dismissive of any questioning of his personal responsibility.
However, a club insider close to today's decision told Reuters the notion that "player power" had played any part in his sacking was simply untrue.
"The decision had been entirely down to the way the team have been playing", the source said.
Mourinho repeatedly said he could not compete with the spending power of rivals City and Liverpool, but he has signed £400 million worth of talent in the last two years.
He splashed out to acquire Pogba, yet has struggled to find a way to make the best use of him. Belgium striker Romelu Lukaku came in from Everton and has had success in fits and starts but Chile international Alexis Sanchez, reportedly the highest-paid player in the league, has been an unmitigated flop.
Having proved himself a defensive master in his previous jobs, even that touch has deserted Mourinho.
For four of the past five years goalkeeper David de Gea has been the United fans' player of season, but even the Spaniard has not been able to paper over the gaping cracks in an ever-changing rearguard this season and he too has looked shaky.
Mourinho's selections and tactical approach brought a regular outpouring of criticism from former players and they were quick to mark his departure.
"If you were looking to go to a club and do everything wrong and get the sack, you would do what he's done," said Jamie Redknapp, while another ex-Liverpool midfielder Graeme Souness said there was an "air of depression" around the club.
Former United captain-turned pundit Gary Neville said the team had been "crawling on their knees" and suggested the 46-year-old Pochettino was the ideal replacement.
"My view is they need someone who meets the three key principles of the football club, and that is promotion of youth, entertaining football, and to win football matches," he said.
There was some sympathy from Liverpool boss Juergen Klopp, however. "He has all my respect, he has had unbelievable success," said the German. "I can imagine the last few months were not a joy for anybody, especially him."
United, who face another 'new money' rival in the shape of Paris St Germain in the Champions League last 16, said they will appoint a caretaker manager in the next 48 hours, with club sources saying that may be someone who served under Ferguson.