The midfielder was sent off for kicking the ball into a crowd of players as referee Mike Jones was breaking up a confrontation between Diego Costa and several West Brom defenders, after Saido Berahino had put Tony Pulis’ men in front early on in the Premier League encounter.
Berahino added a second from the spot shortly after the break after he was felled by John Terry in the box, while Loic Remy fired against the upright before Chris Brunt drilled in the Baggies’ third to seal a comfortable victory.
The result ended Chelsea’s 16-game unbeaten run in the Premier League – the longest by any team in the top flight this season – and means the Blues have now lost three of their last four top-flight visits to The Hawthorns.
It was a fast-paced opening to the game, despite neither side technically having anything to play for other than pride, and Callum McManaman went close with the first opening as he skipped past a defender before firing a shot from a tight angle, only to see the ball whistle across goal via a deflection.
But the hosts soon took the lead as, in the ninth minute, the ball was squared to Berahino, who rolled the ball across to his right and curled a superb 20-yard strike into the bottom corner.
Chelsea had not won on their previous three visits to The Hawthorns, and struggled to find their way through a well-organised home side, but they nearly fashioned an equaliser when Costa played a low cross in from the left and Eden Hazard met it from close range, only to see it take a deflection and loop over the crossbar.
Then came the game’s bizarre moment just before the half-hour mark. Costa and Gareth McAuley were reprimanded for shoving in the box and, while the Chelsea striker was being booked, Fabregas kicked the ball towards the crowd and hit Chris Brunt on the side of the head, prompting referee Jones to show him the red card.
It took some time for the game to calm down, but the pattern of play was similar as both sides had good spells of possession.
After Costa was involved again, this time being shoved to the ground by Jonas Olsson, Chelsea full-back Filipe Luis drilled the resulting free-kick from 20 yards, whipping it around the wall and drawing a firm parry from Boaz Myhill in the middle of his goal.
The hosts regularly threatened on the break, and one such move forward saw James Morrison play a good ball beyond the defence for Brunt, whose low effort across goal was turned away by Thibaut Courtois.
Things certainly failed to calm down after the break, as in the first minute of the second half the hosts were awarded a penalty when Brunt slid a pass through for Berahino, who was brought down by Terry inside the area.
The Chelsea skipper escaped what could have been a red card, but Berahino fired the spot-kick low towards the corner as the hand of Courtois couldn’t stop it going in to double the hosts’ lead.
It looked a long way back for Mourinho’s men, but they nearly pulled one back after a superb spell of passing resulted in Ruben Loftus-Cheek finding Remy, who worked the ball to his left before drilling a low shot which hammered the post and came back out.
Any hope of a comeback was extinguished on the hour mark when, after Courtois tipped over a Morrison effort, West Brom took the corner short to Brunt, who fired a stunning shot from a wide angle and it caught the goalkeeper out, sneaking in at the near post to put Pulis’ men out of sight.
Chelsea continued to enjoy plenty more time on the ball, and Nemanja Matic had a good chance to pull one back as Gary Cahill flicked a corner on at the near post, but the midfielder could only fire straight at Myhill from close range.
It wasn’t to be the visitors’ night, and Mourinho’s men succumbed to their first Premier League defeat since the 5-3 loss to Tottenham on New Year’s Day.
West Brom: Myhill (7), Dawson (7), McAuley (7), Olsson (7), Lescott (6), Brunt (7), Fletcher (7), Yacob (8), Morrison (7), McManaman (7), Berahino (8).
Used subs: Baird (6), Gardner (6), Ideye (6).
Chelsea: Courtois (6), Ivanovic (6), Cahill (6), Terry (6), Luis (5), Loftus-Cheek (7), Matic (6), Remy (6), Fabregas (4), Hazard (6), Costa (5).
Used subs: Ake (6), Cuadrado (5), Brown (6).
Man of the match: Saido Berahino