Match ends, Watford 3, Manchester United 1.
Manchester United suffered their third defeat in a week as late goals from Juan Zuniga and Troy Deeney gave Watford a deserved Premier League win.
The hosts led when Etienne Capoue converted Daryl Janmaat's cut-back, only for Marcus Rashford to level from close range.
Zuniga powered in with seven minutes to go, 53 seconds after coming on.
And Deeney then scored a penalty in injury time after Marouane Fellaini fouled Zuniga in the box.
- Analysis: Man Utd lacking width, a playmaker and a plan
- Watch: MOTD3 - "Somebody needs to be dropped at United"
- Relive Sunday's Premier League action
Mourinho's magic waning?
Following a derby defeat by Manchester City last weekend and the midweek Europa League loss to Feyenoord, Mourinho and United have had a rough eight days.
It is the first time Mourinho has lost three consecutive games in a season since February 2002, when he was Porto boss.
But the self-proclaimed "Special One", who has won eight league titles across Europe and the Champions League twice, has now lost 11 of his past 21 Premier League games in charge of Chelsea and Manchester United.
Mourinho made five changes to his starting line-up from the trip to the Netherlands, with Wayne Rooney and Zlatan Ibrahimovic restored to the side, but United were disorganised and ineffective, particularly in the first half.
The United boss looked unhappy with the home side's opening goal, feeling Anthony Martial was fouled by Miguel Britos, but he will surely need to look more closely at his own side's failings as they drifted six points behind league leaders Manchester City.
How do you solve a problem like Pogba?
Rooney, Ibrahimovic and £89m Paul Pogba all struggled to make an impact on the game and at the back United failed to cope with Watford's energetic pressing and the crosses from wing-backs Janmaat and Jose Holebas.
Pogba, who returned to Old Trafford from Juventus in the summer, continued to look far from being the most expensive player in the world, although he was deployed in a deep midfield role alongside Fellaini that did little to get he best out of the France international.
He did have United's best effort in the first half, hitting a 25-yard dipping shot that struck the crossbar.
But his attacking talents were wasted so far away from the opposition's goal and, after starting the past five games, Mourinho will have to consider dropping Pogba or changing his system to accommodate the 23-year-old.
While most of Mourinho's side toiled, the rise of Rashford continued, as the 18-year-old England striker scored on his first Premier League start of the season, taking his tally to 10 in 23 games.
He even started the move for his goal, playing a one-two with Ibrahimovic, whose cross dropped to Rashford's feet for an easy finish from five yards out.
Hornets boss Walter Mazzarri, who managed Zuniga at Napoli, leapt for joy when the midfielder applied the finish to Roberto Pereya's cut-back with his first touch.
Mazzarri had numerous run-ins with Mourinho during their time in Italy - with former Inter manager Mourinho saying the Italian was a hard-working "donkey" that would "never become a thoroughbred".
Mazzarri, who took over at Vicarage Road in the summer, insisted before the game that he and the Portuguese get on fine now, but he could not hide his delight when his side went 2-1 up and danced on the side of the pitch.
With numerous chances in the first half, including an open goal for Odion Ighalo after keeper David de Gea dropped the ball, Mazzarri's men should have been further ahead at the break, but scored a thoroughly deserved opener when Martial was dispossessed and Janmaat teed up Capoue.
Watford looked like their pressing game had taken the energy out of their performance as United improved in the second half, but they responded late on for their second win of the season and first at home.
Man of the match - Sebastian Prodl (Watford)
What the managers said
Watford boss Walter Mazzarri:
"It is a very important three points to give my players the confidence to go ahead.
"We were playing well, we conceded a goal and we stop playing. We were afraid then we made the changes and everything went the right way - it is something we need to work on.
"I don't look too much at results but how we play. We have been improving dramatically."
And what about a drink with Mourinho after the game? "If he has time we will do it. We spoke before the game - even one day next week we can go out for dinner."
Manchester United boss Jose Mourinho:
"I reflect on three factors from the match, but only one of them I can improve.
"The first factor depends on ourselves, it relates to our individual mistakes and collective mistakes as individual players and as a team. We have to improve something, it is in our hands.
"At 1-1 everyone thinks we are going to win the game. We were showing complete control, intensity, creation.
"But their second goal is a mistake that goes against our plan and our training, because our intention was for their wing-backs to be pressed and not let them progress.
"And what happened was the guy gets the ball 20-25 metres away from our box and instead of being pressed, we give him the space to progress. Nordin Amrabat receives the ball and our left-back Luke Shaw is 25 metres from him instead of five.
"The second factor is the referee and I can't control their mistakes.
"The third factor is luck, we didn't have it. We were the best team when we lost."
Rashford's scoring record and other stats you need to know
- Rashford (10) has scored more goals in all competitions for Manchester United than any other player since his debut in February.
- Capoue has scored four goals from just five shots on target in the Premier League this season.
- Zuniga's goal was the fastest by a substitute since Liverpool striker Divock Origi's (37 seconds) against Aston Villa in February.
- Since his debut in August 2014, Daryl Janmaat has provided more assists in the Premier League than any other defender (11).
- Manchester United scored their 800th Premier League away goal; 96 more than any other team.
- 3BritosBooked at 77mins
- 22JanmaatSubstituted forN Amrabatat 51'minutes
- 29CapoueSubstituted forZuñigaat 82'minutes
- 25HolebasBooked at 77mins
- 9DeeneyBooked at 86mins
- 24IghaloSubstituted forSuccessat 90'minutes
- 7N Amrabat
- 1de Gea
- 25A ValenciaSubstituted forMataat 62'minutes
- 23ShawSubstituted forDepayat 85'minutesBooked at 89mins
- 27FellainiBooked at 88mins
- 6PogbaBooked at 75mins
- 11MartialSubstituted forYoungat 38'minutes
- 10RooneyBooked at 90mins
- Michael Oliver
- Shots on Target
Second Half ends, Watford 3, Manchester United 1.
Goal! Watford 3, Manchester United 1. Troy Deeney (Watford) converts the penalty with a right footed shot to the top right corner.
Delay over. They are ready to continue.
Delay in match Juan Zuñiga (Watford) because of an injury.
Penalty conceded by Marouane Fellaini (Manchester United) after a foul in the penalty area.
Penalty Watford. Juan Zuñiga draws a foul in the penalty area.
Chris Smalling (Manchester United) wins a free kick in the defensive half.
Foul by Troy Deeney (Watford).
Wayne Rooney (Manchester United) is shown the yellow card.
Foul by Wayne Rooney (Manchester United).
Roberto Pereyra (Watford) wins a free kick on the right wing.
Corner, Watford. Conceded by Wayne Rooney.
Corner, Watford. Conceded by Memphis Depay.
Foul by Zlatan Ibrahimovic (Manchester United).
Miguel Britos (Watford) wins a free kick in the attacking half.
Substitution, Watford. Isaac Success replaces Odion Ighalo.
Memphis Depay (Manchester United) is shown the yellow card.
Foul by Memphis Depay (Manchester United).
Nordin Amrabat (Watford) wins a free kick on the right wing.