An acrimonious exit from Chelsea, troubling beginnings to his time at Old Trafford. Here, Sportsmail looks at five things that Mourinho has to fix.
ROONEY'S RULES
Back in July, Mourinho scoffed at the idea of Wayne Rooney playing a deeper role in midfield.
The Portuguese joked that if he had as much time on the ball he could play wonderful long passes too. But Rooney has seemingly appointed himself to a free role for United and England and it is becoming detrimental to both.
England manager Sam Allardyce admitted he could not control the player but Mourinho has to if he does not want to keep seeing moves from defence into attack break down.
Rooney has one goal this season in a Bournemouth game that he played poorly in. His touch has often been woeful, many of his passes astray. He has lost the sharpness that made him once lethal. He has three yellow cards in his last three United games.
Rooney is a 30-year-old who has spent his entire career in and out of shape.
He is a year younger than Cristiano Ronaldo, but with the physiology of a man many years his senior. Mourinho needs to drop him, or tell him what to do out on the pitch.
WHERE ARE THE GREAT CENTRE-BACKS?
United have been defined as much in history by their great centre back pairings as they have their memorable attackers.
Martin Buchan and Gordon McQueen, Steve Bruce and Gary Pallister, Jaap Stam and Ronny Johnsen, Rio Ferdinand and Nemanja Vidic.
Could Chris Smalling claim a place in that category? He had potential to once but he is nowhere near realising that.
Eric Bailly, the £30million signing from Villarreal, looks like he could. Alongside Daley Blind the pair conceded only twice in their opening four victories.
Blind, a cultured ball-player who can read the game well, complemented Bailly's slightly naive, but still effective, gung-ho approach.
But the Dutchman was dropped for Smalling after the City defeat and United have lost twice more.
Against Watford, Sebastian Prodl played a long ball over the top and Smalling collided with David de Gea to gift Odion Ighalo an open goal.
Even though the Watford striker fluffed the chance, the incident did little to inspire confidence and showed the United defence was there for the taking.
THE FEARSOME FACTOR
When United used to take to the field in the late '90s through to the '00s, at the peak of the Sir Alex Ferguson glory days, opponents would spend the final few seconds before kick-off wondering how bad the damage would be that afternoon.
Publicly, yes, they would say they had every chance of winning the game. Privately, they were quaking. United were that good back then. Now, they are just another Premier League team.
Catch them on a bad day - nay, even an average day - and anyone in the league can beat them. That aura will take years to regrow and may even be impossible to capture once again.
STOP HAMMERING THE PLAYERS
Mourinho's biggest mistake at Chelsea - bar his ugly one-man crusade against their team doctor - was forcing the players to turn against him by repeatedly blaming them for mistakes.
He is continuing that approach at United. He singled out Luke Shaw after the Watford game, criticising the left back for his defending that led to the second goal.
'For their second goal it is a mistake that goes against our plan and against our training, because our intention was for their wing-backs to be pressed and not let them progress,' Mourinho said.
'And what happened was the guy gets the ball 20 to 25 metres away from our box and instead of being pressed, we give him the space to progress.
'And when he was inside the box we were not confident enough to tackle. For the second goal, Amrabat receives the ball and our left back is 25 metres from him instead of five.
'But even at 25 you have to jump and go and press. But no, we wait. This is a tactical but also a mental attitude. It's something that doesn't become perfect in a couple of weeks. So we have to improve, no doubt.'
Mourinho lost the dressing room in the latter stages of his time at Stamford Bridge.
Players who had fought and battled their way to the Premier League title the season before were no longer prepared to put it all on the line for their constantly critical manager.
GETTING THE BEST OUT OF POGBA
One must maintain a sense of perspective with Paul Pogba; he has played five games for Manchester United since his £89m move from Juventus.
He is 23 years old. He reached the Euro 2016 final with France.
There are not many players who made it to that stage and are not feeling the lagging effects at the start of this season.
But still, he cost a world record fee and Mourinho will know that he needs his star central midfielder to start paying something back.
Maybe with an assist, a goal, or just a scything through pass, anything to allay unease that an awful lot of money has been spent on a player who is not going to score 20-plus goals per season.
