The former Arsenal midfielder was given his marching orders in bizarre circumstances as the young striker scored twice before Chris Brunt sealed the three points
Saido Berahino gave the Baggies the lead as he curled a shot into the bottom corner from the edge of the area before Fabregas was dismissed in bizarre fashion.
The midfielder kicked the ball against Chris Brunt's head as referee Mike Jones dealt with an altercation between Diego Costa and Gareth McAuley, with Jones wasting no time in giving Fabregas his marching orders.
Berahino doubled the hosts' lead less than two minutes into the second half as he converted from the penalty spot after he was brought down by John Terry.
Loic Remy hit the post as the Premier League champions looked for a route back into the game, but Tony Pulis' side secured the three points when Brunt's powerful drive beat Thibaut Courtois at his near post.
Chelsea boss Jose Mourinho was gushing in his praise of West Brom counterpart Pulis in the build-up to the match – hailing his "absolutely amazing" record – and the Welshman certainly had his players fired up for the visit of the champions.
In a lively start, Courtois got down well to turn Callum McManaman's angled drive behind and Fabregas evaded two tackles as he slalomed into the penalty area at the opposite end before firing straight at Boaz Myhill.
Berahino took just nine minutes to repay Pulis' decision to recall him by putting Albion ahead in some style.
Joleon Lescott rolled a simple pass that Berahino turned and ran on to before escaping the attentions of Gary Cahill and rifling right-footed into Courtois' bottom left-hand corner from 25 yards out.
An inswinging Brunt corner midway through the first half required Courtois to punch clear right under his crossbar and Chelsea broke immediately. Costa, back after over a month out with a hamstring injury, worked his way down the left flank and picked out Eden Hazard at the near post, but his close-range shot looped over the bar off Lescott.
Then came the flashpoint that saw Fabregas dismissed in unusual circumstances.
McAuley and Costa were involved in an off-the-ball tangle in the penalty box that resulted in the latter being booked and play being stopped as the two sets of players voiced their views at referee Mike Jones.
Fabregas, all on his own on the opposite side of the penalty area, inexplicably struck the dead ball back into the crowd of players and into the back of Brunt's head, prompting Jones to brandish a straight red.
Filipe Luis stung Myhill's palms with a free-kick that the Albion goalkeeper saw late and Courtois saved smartly with his feet to deny Brunt from James Morrison's throughball as an eventful opening 45 minutes came to a close.
The drama continued straight after the interval as Terry bundled Berahino over in the box to concede a penalty within 30 seconds of the restart.
A full-stretch Courtois got a hand to Berahino's spot-kick, but it nestled in the bottom corner to make it 20 goals for the season in all competitions for the England Under-21 international and 14 in the league.
Loic Remy struck the post from the edge of the area with Myhill well beaten 10 minutes later, but any hopes of a fightback were dashed by Brunt, who cut in from the right following a short-corner routine and lashed low past Courtois at his near post.