Olivier Giroud, an £18million January signing from Arsenal, came on for his Chelsea debut and the Blues pushed for an equaliser - getting one through a smart Eden Hazard strike.
The Hornets - who had won only one of their previous 12 Premier League games - led controversially as Gerard Deulofeu went down easily under keeper Thibaut Courtois' challenge and Troy Deeney scored the penalty.
Daryl Janmaat struck first, in the 84th minute, followed in the 88th by Deulofeu, while Roberto Pereyra delivered the final blow in stoppage time.
Footballers often try and give off a 'macho attitude' but the passionate young Brazilian clearly doesn't feel that this side of the game is particularly important.
MF Victor Moses, 5 - Failed to put a single decent cross into the box despite numerous opportunities.
Much more of this and the divorce anticipated for the summer would surely be brought forward. "The starting XI was wrong because our start was very poor and it was difficult to change something". With speedy wingers streaming up on either side of the centre-forward, the counter is on.
And it turned out that João Castelo Branco managed to speak to Richarlison in an exclusive post-match interview, in which the 20-year-old could explain his feelings and what he's been through lately. Not introducing the Frenchman once reduced to 10 men was equally baffling. Apart from that struggled to get on the ball for most of the game.
MF Tiemoue Bakayoko, 2 - Could not do right for doing wrong, inadvertently creating two clear goal-scoring chances for Watford. I think I was good in the game.
The former boss has been very outspoken on the lack of support he has received from the clubs hierarchy.
However, Courtois had few options on the ball.
Chelsea had done well to stay in the contest as long as they did but they always lacked cohesion.
Watford manager Javi Gracia explained why he took off Richarlison, despite his impressive performance against the Premier League champions.
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