Ole Gunnar Solskjaer was able to call upon the services of Paul Pogba again from the bench for the first time since September, but by the time he was introduced the game was already beyond United, who have made a habit of dropping points against teams in the bottom half.
A complete lack of cohesion plagued the Red Devils in the first half, with Jesse Lingard wasting their only genuine opportunity as their front four made a minimal impact.
Helped by a shambolic error from David de Gea, Watford got the lead in the 50th minute through Ismaila Sarr and soon made it 2-0 thanks to a Troy Deeney penalty, giving Nigel Pearson a first win as Watford boss.
Oh dear David De Gea, Watford take the lead! pic.twitter.com/m8xN7yhWXD— NBC Sports Soccer (@NBCSportsSoccer) December 22, 2019
A first half sorely lacking in quality failed to produce many clear-cut opportunities, but Watford thought they opened the scoring in the 18th minute when Abdoulaye Doucoure smashed home, only for referee Lee Mason to somewhat harshly deem Craig Cathcart fouled De Gea.
Lingard was then guilty of a woeful miss just past the half-hour mark, electing to chip Ben Foster when through on goal and his effort landed on the roof of the net.
United found themselves trailing just after the restart, though Watford needed a De Gea howler to score – Sarr's tame volley bouncing up at the Spaniard and he failed to stop it squirming into the top corner.
WATFORD LEAD BY 2 😳 pic.twitter.com/BJzAyr6xjp— NBC Sports Soccer (@NBCSportsSoccer) December 22, 2019
Another lapse – this time from Aaron Wan-Bissaka – allowed Watford to extend their lead four minutes later, as the right-back clumsily tripped Sarr and Deeney smashed his spot-kick down the middle.
Pogba was introduced for Lingard just past the hour mark and showed touches of class, but he was unable to inspire a turnaround.
What does it mean? The same old problems for United
The inconsistent nature of United's form continues to baffle. Although often effective against the best teams in the division, Solskjaer still has not worked out how to break down many of the Premier League's so-called lesser sides.
An ordinary Watford team that is deservedly bottom of the table should have been straightforward opposition for United, yet at no point did they look even remotely capable of getting a win on Sunday.
A Sarr is born
Perhaps Sarr was more functional than barnstorming, but it was undoubtedly an encouraging display from the young winger. It was his shot that brought about De Gea's error and then he won the penalty from which Deeney scored. You cannot expect much more than that from him.
De Gea becoming a target
De Gea's form has underwhelmed of late and this was almost certainly the worst performance of the lot. He was extremely fortunate to win a free-kick in the first half when Watford thought they had scored, and his error for Sarr's goal was inexcusable.
United return to Old Trafford on Boxing Day as they host Newcastle United. Watford go to Sheffield United the same day.