Ashley Young scored twice in six first-half minutes as United hit four on the road at Watford and kept up the pressure on unbeaten leaders Manchester City.
Anthony Martial also scored before the interval as the Reds raced into a three-goal lead only for Watford to hit back - but Jesse Lingard's sublime individual goal rubber-stamped the result and moved United back to within five points of City, who host Southampton on Wednesday.
Young continued his superb recent form when he put us ahead with the first attack of the game on 19 minutes. Having scored with a heavily deflected effort against Brighton on Saturday, it was a far more conventional finish this time, when Lingard's cross picked out Young and he half-volleyed in beyond Heurelho Gomes.
Five minutes later, Young stepped up to the mark with a stunning free-kick that would not have looked out of place in the Cristiano Ronaldo blueprint. Paul Pogba went down under a challenge from Abdoulaye Doucoure, and Young beautifully blasted home the set-piece with aplomb from 25 yards.
Marco Silva's side had come into the match on the back of two successive wins which had lifted them to eighth in the table, but the Hornets were left stung when Martial looked to have put the result beyond doubt just after the half-hour mark.
United broke from midfield and Romelu Lukaku flicked the ball on into the path of the Frenchman, who fired in his eighth goal of the season from close range.
The hat-trick hunting Young had a chance to seal his unlikely treble, but this time Watford held firm as another free-kick from an almost identical position failed to clear the wall. United would have gone in at half-time even further ahead if not for the strong reactions of Gomes, who twice saved well from Lukaku and then the lurking Lingard in quick succession.
Richarlison headed over the bar for the hosts, but United managed the game superbly and rarely looked in danger of squandering the lead, until we were stung by the Hornets in an unwelcome twist in the tale.
Marcos Rojo, on his first league start for seven months, clumsily felled Roberto Pereyra and Watford substitute Deeney - back after a three-match ban - sent David de Gea the wrong way from the spot.
For the first time in the contest, United were pegged back and the three-goal advantage had soon been reduced further as Watford moved to within a goal of the Reds when Abdoulaye Doucoure poked in the loose ball from close range, but any hopes of an incredible comeback proved to be short-lived.
Two minutes later, Lingard re-established United's two-goal lead with an incredible solo goal (are you watching, Messi), when he collected the ball deep inside his own half, slalomed beyond the back-pedalling Watford defence and smashed in a composed finish to seal the points.
United Faithful Man of the Match: Ashley Young
Overall team performance: 7.5/10