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, which has 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.
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.
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.