Manchester United travelled to Vicarage Road hoping to avoid a third defeat in as many games, but the Red Devils were stung by the Hornets, as Watford recorded a comfortable 3-1 victory.
José Mourinho made a host of changes to the side that was beaten by Feyenoord in midweek, but the 4-3-3 formation from Rotterdam was retained, as Wayne Rooney joined Paul Pogba and Marouane Fellaini in the midfield three, with Marcus Rashford making his first Premier League start of the season out wide.
And United benefitted from some astounding profligacy from Odion Ighalo after just 12 minutes, when the Nigerian striker shot wide of an open goal, having been presented with a gift of a chance due to a communication breakdown between Chris Smalling and David de Gea.
De Gea atoned for his mistake minutes later to acrobatically deny Troy Deeney’s close range header.
The Red Devils gradually worked their way into the game, and on 30 minutes Paul Pogba came within inches of opening his United account when he rattled the Watford crossbar with a beautifully struck swerving effort from 25 yards.
But soon after, the home side took the lead through the in form Etienne Capoue. Anthony Martial — who was still recovering from an earlier clash of heads with Daryl Janmaat — was robbed of possession, although the Frenchman felt he was fouled by Miguel Britos. Janmaat raced to the byline and cut back for Capoue to finish.
Martial, visably dazed, was unable to continue and was replaced by Ashley Young after 38 minutes.
Into the second half, United continued to struggle to get a grip on the game, and after an hour, Mourinho made an attacking change by bringing Juan Mata on for Antonio Valencia.
Seconds later, the away side were level. Zlatan Ibrahimovic came deep to collect the ball and played a one-two with Rashford. The Swede moved into the penalty area and crossed towards Rashford, who reacted quickly when his initial header was blocked to poke the ball into the net from close range.
The momentum was with United as they pushed for a second, and with 12 minutes to play, Heurelho Gomes made a brilliant reaction save to keep out a powerful Ibrahimovic header.
But, in the 84th minute, it was Watford who took a late lead against the run of play. Roberto Pereyra and Nordin Amrabat combined down the right, and substitute Juan Zuniga swept home a fantastic curling finish from 15 yards out.
United’s misery was further compounded when, deep in stoppage time, the home side were awarded a penalty. The magnificently-named Isaac Success, fresh of the bench, ran away from two Reds’ defenders before cutting back to Zuniga, who was fouled in the box by Fellaini.
Watford captain Deeney smashed home the penalty to make in 3-1 to the Hornets, and three defeats in a row for Mourinho’s men.