The ramifications will be nothing like as severe for Roberto Di Matteo as they were for André Villas-Boas, who was sacked after Chelsea lost here in March, but this still felt like a significant defeat for the Italian.
It is now four games without a win for Chelsea in the Premier League – their worst run since Di Matteo took over – and there will be inevitable comparisons with previous seasons, when the onset of winter has frozen Chelsea's title aspirations.
Having picked a team with what appeared to be one eye on Tuesday's Champions League game against Juventus in Turin, Di Matteo was forced to endure the sight of West Bromwich Albion, his former club, continue their remarkable start to the season with a sixth home win in seven matches that lifts them to the dizzy heights of fourth place.
Di Matteo introduced Oscar and Mata in the second half but by that point Albion were 2-1 in front after Peter Odemwingie had headed in Shane Long's cross. Long, who left the field to a standing ovation when he was substituted 10 minutes from time, was outstanding. He scored the goal that put Albion in front earlier in the game and caused Gary Cahill and David Luiz problems throughout with his tireless running.
Chelsea drew level through Eden Hazard's header, which glanced off Billy Jones, but they were unable to find a second equaliser following Odemwingie's fourth goal of the season. Daniel Sturridge was frustrated on several occasions by the excellent Boaz Myhill but he should have done better three minutes from time, when he drilled across the face of goal following Oscar's sublime lay-off.
Albion's opener came from their first meaningful attack of the game, and from Chelsea's point of view it was a poor goal to concede.
Released by Zoltan Gera's clever cut back, James Morrison had the time and space to deliver a cross that Long headed beyond Petr Cech from inside the six-yard box. Luiz, taking the place of the injured John Terry in central defence, should have been marking Long but his poor positioning was ruthlessly punished by the Irishman.
Although Chelsea had come close to scoring a couple of times in the seventh minute, when Liam Ridgewell cleared Ryan Bertrand's shot off the line and Hazard had a decent penalty shout turned down, there was lack of penetration about their play at times in the first half.
"Attack, attack, attack" chanted the travelling supporters after seeing Chelsea fail to turn long periods of possession into chances.
That changed six minutes before the interval, however, when Chelsea equalised. Hazard floated a pass out to Cesar Azpilicueta and the full-back promptly returned the favour, crossing deep towards the far post where Jones was caught ball-watching. By the time Jones got his bearings it was too late, Hazard's free header deflecting off him and past Myhill at the near post. Victor Moses could and possibly should have added a second on on the stroke of half-time but, teed up by Cahill, the former Wigan winger blazed high and wide.
Albion were not so generous. Six minutes into the second half the home team restored their advantage when Long's superb cross was headed home by Odemwingie, who beat Bertrand to the ball at the near post. Di Matteo responded by throwing on the cavalry, Oscar and Mata summoned from the bench to replace Oriol Romeu and the hugely disappointing Fernando Torres. The changes almost brought immediate reward, Mata's glorious pitching wedge of a pass inviting Sturridge to toe-poke a shot that Myhill did well to save. The Albion goalkeeper produced an even better stop to deny the same player in the 78th minute, when he stuck out a hand to deny Sturridge after the forward burst through the middle.
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