Match ends, Watford 1, Manchester United 2.
Manchester United boss Jose Mourinho said Romelu Lukaku epitomised his team's performance as they withstood a spirited fightback from Watford to claim back-to-back victories and condemn the Hornets to a first loss of the season.
United scored twice in three first-half minutes as Belgium striker Lukaku diverted Ashley Young's cross in via his chest before Chris Smalling fired home Marouane Fellaini's knock-down.
Watford pulled a goal back midway through the second half through Andre Gray, but despite putting Mourinho's side under heavy pressure for the final 20 minutes, they were unable to find an equaliser.
Nemanja Matic was sent off after a mistimed challenge on Will Hughes saw him pick up his second booking in the closing minutes, but United were able to hold on to claim their 15th victory over Watford in their past 16 meetings.
Mourinho was particularly impressed with Lukaku's defensive work-rate, after the striker chased down and tackled Watford substitute Adam Masina in the right-back position late on.
"I think that last Romelu Lukaku tackle as a right-back shows everything," Mourinho said.
"I think he is the image of the team, a team that played very well in long periods in the first half. But we had the experience also here last season where they don't give up and when the game is open they believe."
The result leaves Watford fourth, while United climb to eighth.
Pogba the heartbeat for United
The Hornets had the better of the opening 20 minutes, Troy Deeney forcing a superb save from David de Gea, but United grew in confidence immeasurably after their quick-fire double strike.
Lukaku's fourth league goal of the season came in bizarre fashion as he turned home Young's cross with his lower chest.
In contrast, Smalling showed the technique and poise of a seasoned striker to bring down Fellaini's knock-down on his chest and volley into the roof of the net.
However, while Belgium midfielder Fellaini was involved in both goals, it was a Frenchman who was United's heartbeat.
On his 100th appearance for the club, Paul Pogba played with real confidence - testing Foster twice from distance in the first half, before being brilliantly denied on the brink of half-time.
Alexis Sanchez also impressed and did have the ball in the net on 24 minutes, only for his effort to be ruled out for offside, but United were made to work for the win in the second half as Watford grew into the game.
Sanchez and second-half substitute Anthony Martial both had opportunities to make the result safe in late counter-attacks as Watford pressed for an equaliser, but fine interventions by Craig Cathcart and Christian Kabasele denied them.
However, United were not made to pay for those missed chances, as their backline stood firm to close out a hard-earned win and three points.
Watford's perfect run comes to an end
It was always going to be difficult for Watford to carry the momentum built during their first four matches over the international break, and that proved to be the case.
Despite a promising opening for the hosts, which saw Abdoulaye Doucoure strike at goal after just three minutes, United's first-half goals dampened the Hornets' spirit.
Their central midfield pairing of Doucoure and Etienne Capoue were overrun by United's midfield three in the first half, which left Deeney and Gray starved of service.
But, with the game stretched in the second period, Watford were able to get the ball to their creative players more often, meaning the likes of Will Hughes and Roberto Pereyra were able to influence the game.
And it was Doucoure who created Watford's goal, as after a well-worked move he was able to find an unmarked Gray, who applied an emphatic finish to score his second of the campaign.
The Hornets spent much of the remainder of the game camped in United's half, applying relentless pressure, although chances were still hard to come by.
But De Gea kept out another Doucoure long-range effort, and then pulled off a trademark spectacular save from Kabasele's header deep into injury time.
Man of the match - Marouane Fellaini
'I am sorry we don't have a camera in our dressing room' - what they said
Manchester United manager Jose Mourinho told BBC Sport: "With this win we keep the same points as Spurs and Arsenal. We have that distance of four and six points to the other top teams - if we don't win this match obviously that distance goes a little more.
"They are a good team, they know how to create problems to the top teams. And I told the players at half-time - sometimes I am sorry we don't have a camera in the dressing room because I told them what exactly happened - that it could be a match to win four or five nil, or to be in real trouble. In the second half we decreased our intensity, then the goal happened, then the game is open."
Watford boss Javi Gracia on BBC Sport: "We were much better in the second half than in the first. United played better in the first half; they created more chances and had more possession. In the second we were able to take a step forward to have more possession and to create more chances. In the last seconds we had two good chances to score. Today we don't get the points but I can be proud of the second half.
"In the first half we conceded two goals, after that the game was tough, we needed to press higher. We suffered some difficult situations, but we had to try and score two goals. It was a pity."
Lukaku's happy hunting ground - the stats
- All three of Romelu Lukaku's Premier League goals against Watford have been at Vicarage Road.
- The Belgium striker has scored three and assisted two in six Premier League appearances versus Watford.
- Manchester United have been shown a red card in consecutive league games for the first time since September 2014.
- Romelu Lukaku has scored 20 goals in 39 Premier League apps for Manchester United; only three players have reached 20 goals for the Red Devils in fewer games than the Belgian - Ruud van Nistelrooy (26), Robin van Persie (32) and Dwight Yorke (34).
- Watford have lost 13 of their last 14 matches against Manchester United in league competition, with their solitary win coming at Vicarage Road in September 2016 (3-1).
- Jose Mourinho has won six of his seven Premier League encounters with Watford (one loss), including each of the past four.
- Chris Smalling has ended up on the winning side in all 11 Premier League games in which he has scored, the joint-best such record in the competition (also 11 games for Ryan Babel).
Manchester United begin their Champions League campaign on Wednesday with a trip to Switzerland to face Young Boys (20:00 BST), before hosting Wolves at Old Trafford on Saturday 22 September (15:00 BST).
Watford are next in action when they travel to Fulham on the same day (12:30 BST).
- 2JanmaatSubstituted forFemeníaat 72'minutes
- 15CathcartSubstituted forSuccessat 88'minutes
- 25HolebasBooked at 36minsSubstituted forMasinaat 84'minutes
- 29CapoueBooked at 34mins
- 1De Gea
- 25A ValenciaBooked at 81minsSubstituted forBaillyat 90+2'minutes
- 31MaticBooked at 90mins
- 14LingardSubstituted forMartialat 71'minutes
- 7SánchezSubstituted forMcTominayat 84'minutes
- Mike Dean
- Shots on Target
Second Half ends, Watford 1, Manchester United 2.
Corner, Watford. Conceded by David De Gea.
Attempt saved. Christian Kabasele (Watford) header from the centre of the box is saved in the bottom right corner. Assisted by Will Hughes with a cross.
Second yellow card to Nemanja Matic (Manchester United) for a bad foul.
Foul by Nemanja Matic (Manchester United).
Will Hughes (Watford) wins a free kick on the right wing.
Substitution, Manchester United. Eric Bailly replaces Antonio Valencia.
Attempt blocked. Will Hughes (Watford) left footed shot from the right side of the box is blocked. Assisted by Kiko Femenía.
Attempt saved. Abdoulaye Doucouré (Watford) left footed shot from outside the box is saved in the bottom left corner. Assisted by Roberto Pereyra.
Substitution, Watford. Isaac Success replaces Craig Cathcart.
Paul Pogba (Manchester United) wins a free kick in the defensive half.
Foul by Etienne Capoue (Watford).
Substitution, Watford. Adam Masina replaces José Holebas.
Substitution, Manchester United. Scott McTominay replaces Alexis Sánchez.
Attempt blocked. Andre Gray (Watford) right footed shot from outside the box is blocked. Assisted by Kiko Femenía.
Antonio Valencia (Manchester United) is shown the yellow card for hand ball.
Hand ball by Antonio Valencia (Manchester United).
Corner, Manchester United. Conceded by Abdoulaye Doucouré.
Corner, Watford. Conceded by Ashley Young.