It is the first time in over a decade the Portuguese has endured such a run as Watford deservedly ran out 3-1 winners on Sunday to add to a home loss to Manchester City the previous weekend and then a Europa League defeat at Feyenoord on Thursday.
So why have Mourinho's men have been misfiring in recent matches.
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There have been questions raised over the longevity of Wayne Rooney's career after the United and England skipper has seen his form dip over the past 12 months.
He may be England's all-time leading goalscorer, and he is also closing in on that accolade at Old Trafford, but he appears a shadow of the teenage sensation that burst onto the scene in 2002.
All those miles in his legs are starting to take their toll and Mourinho has already played his captain in the number 10 role and deeper in midfield in an attempt to utilise his ability. But he was arguably the worst player on the pitch at Watford, spraying balls out of play and misplacing simple passes.
POGBA MAY BE BACK BUT HE LEFT HIS AURA IN ITALY
United's prolonged recruitment of Paul Pogba - a player they allowed to leave for a nominal fee in 2012 - over the summer ended when they paid a world-record amount to bring him back.
Much fanfare was made of his return, a glitzy unveiling for the social media age and plenty of talk of just how much he had improved since leaving under a cloud during Sir Alex Ferguson's tenure.
But so far he has not shown his obvious class for the Red Devils - he went missing for much of the derby defeat and, even when shifted to the number 10 role he covets for the trip to Holland, he fell flat.
There have been flashes of his brilliance, a superb 25-yard effort that cannoned off the Watford crossbar a fine example, but for a player who cost more than Cristiano Ronaldo or Gareth Bale, Mourinho needs him to come good. And quick.
RASHFORD THE ONLY BRIGHT SPARK IN ATTACK...
Having marked his surprise arrival into the United first-team with a hatful of goals last season, Marcus Rashford found himself on the fringes during Mourinho's early weeks.
The 18-year-old was patient and scored a last-gasp winner at Hull before earning his first league start at Watford.
There again he was the man to get on the score-sheet as even Zlatan Ibrahimovic, another much-heralded summer signing, was easily kept on the periphery by Watford's defence.
He is currently the sharpest and brightest option Mourinho had but even his bags of promise will not always be enough.
AND ALL IS NOT WELL AT THE BACK
Mourinho singled out Luke Shaw for criticism after Sunday's loss, claiming he had been drilling his defence to close the gaps to Watford's wing-backs and had not seen that play out on the pitch.
He has also opted to break up what appeared to be a budding defensive partnership between Daley Blind and Eric Bailly, dropping the former in the past two matches in favour of Chris Smalling.
Although Blind and Bailly were out of sorts in the derby, they looked to be forming an understanding while Smalling's return has not changed fortunes with two defeats in two.
David De Gea continues to prove why he is one of the world's best goalkeepers but is often left too exposed.
A FEW TOUGH CALLS HAVE GONE AGAINST UNITED
Whisper it, but Mourinho is right to apportion some of the blame to referees.
He has yet to suggest there is a 'campaign' against his side as he did during his second spell at Chelsea, but there have been a few decisions which have irked the 53-year-old.
United could have easily had a penalty in the derby when goalkeeper Claudio Bravo slid into a challenge with Rooney, while Feyenoord's goal on Thursday came from an offside position.
Mourinho also felt Anthony Martial was fouled in the build-up to Watford's opener - "We are punished by these mistakes and I can't do anything," he said after the Vicarage Road defeat - and he may have a point.