Goals from Etienne Capoue, Juan Zuniga and Troy Deeney, with a late penalty, saw Watford to a comfortable win at Vicarage Road.
Manchester United, whose lone goal came from teenage striker Marcus Rashford, suffered a third straight defeat under manager Jose Mourinho.
It’s not the first time the Portuguese tactician has been on the wrong end of an upset though.
Here, we look at five of his worst defeats in the Premier League to lesser opponents.
MIDDLESBROUGH 3-0 CHELSEA
At the Riverside Stadium in February 2006, Chelsea were stunned in a 3-0 defeat.
Fabio Rochemback grabbed the opener after only 78 seconds before Stewart Downing and Yakubu scored as well.
ASTON VILLA 2-0 CHELSEA
Chelsea’s 18-game unbeaten streak in the league came to an end at Villa Park in September 2007.
Zat Knight and Gabriel Agbonlahor helped push Martin O’Neill’s Aston Villa side over the line in a 2-0 win.
EVERTON 3-1 CHELSEA
Steven Naismith terrorised Chelsea with a hat-trick as he led Everton to a 3-1 win at Goodison Park.
CHELSEA 1-3 SOUTHAMPTON
The champions went ahead through Willian’s free-kick but Steven Davis bagged the equaliser before Sadio Mane pounced to put Saints ahead.
Graziano Pelle then sealed victory for Southampton and condemned Chelsea to their fourth defeat of the season, leaving them in 16th place.
CHELSEA 0-1 BOURNEMOUTH
Glenn Murray’s late winner for Bournemouth shocked Stamford Bridge.
The 82nd-minute goal cranked up the pressure on Mourinho and eventually lead to the end of his second spell with the club.