The hosts had won all four of their league outings before encountering United, who ended that 100 per cent start thanks to goals from Romelu Lukaku and Chris Smalling. The reply came from Andre Gray.

Lukaku's point-blank finish with his midriff opened the scoring in the 35th minute and three minutes later Smalling had the Red Devils roaring.

Former United goalkeeper Ben Foster made a string of saves to keep the scoreline down and Watford gave themselves the chance of salvaging a result through Gray's second goal of the season.

They came up short though in their quest for an equaliser, despite going close after Matic was dismissed in the final moments.

Mourinho could not call on the suspended Marcus Rashford - a player about whom he launched a lengthy defence during his pre-match press conference.

And, while Hornets boss Javi Gracia named an unchanged team for the fifth time, Mourinho opted against playing Luke Shaw - the left-back rested completely after suffering concussion when playing for England.

His replacement, former Watford winger Ashley Young, scored twice in a United victory here last season and played his part again.

De Gea made the first telling stop of the evening, tipping a Troy Deeney shot over the crossbar after Smalling was caught out by a long ball.

The visitors thought they had taken the lead at the midway point of the first half but Alexis Sanchez was flagged offside as he controlled and finished Matic's cross.

The Chile forward came close again on the half-hour mark, testing Foster with a drive, and United hit the front soon after.

After a free-kick had finally been cleared by the hosts, Young collected the ball and crossed for Lukaku who was free to guide the ball home from close range.

The lead could have been doubled almost immediately with Foster again called into action, this time pushing a Paul Pogba strike over.

But United would not be made to wait much longer as another Pogba shot from the resulting corner was blocked behind.

This time Young's corner was sent deep into the box, with Marouane Fellaini's knock-down skilfully controlled on the chest by Smalling, with the defender then arrowing the ball past Foster from six yards.

Mourinho's men should have been toasting a third goal on the stroke of half-time but Foster again kept Watford in it, throwing his body in front of Pogba's close-range strike.

The hosts responded well after the interval and halved the arrears as Gray was left free in the middle of goal to turn home Abdoulaye Doucoure's low cross with 25 minutes remaining.

Watford were the livelier of the two sides but were leaving gaps as they chased the game - Foster and Craig Cathcart combining to prevent Sanchez tapping in a breakaway third for United.

De Gea gratefully kept hold of a well-struck Doucoure effort as the game headed into stoppage time, where Matic collected a second yellow card for fouling Will Hughes.

From the resulting free-kick De Gea had to be at his best to turn away Kabasele's goal-bound header as United held on to claim the points.

- PA