Chelsea midfielder Cesc Fabregas was sent off in bizarre circumstances as the below-par Premier League champions, playing with 10 men for over an hour, slumped 3-0 at West Brom.
Saido Berahino struck twice for Albion but the main talking point at the Hawthorns was the first-half red card shown to Spaniard Fabregas for unsportsmanlike conduct by referee Mike Jones.
The official was in the process of booking Chelsea striker Diego Costa after he tussled with Gareth McAuley when Fabregas, standing some 20 metres away, chipped the ball towards a crowd of players and it hit West Brom's Chris Brunt on the head, prompting Jones to walk over and brandish a red card.
"What Fabregas did didn't hurt anyone, he shouldn't do it but I don't think that the red card was fair," Chelsea manager Jose Mourinho told the BBC.
The Portuguese said his side, who won the title earlier this month with three games to spare, had lost "concentration, intensity and motivation" with nothing to play for.
Chelsea are eight points clear of City going into their final match of the season at home to Sunderland on Sunday.
Keeper Thibaut Courtois prevented Chelsea going into halftime further behind but West Brom won a penalty 29 seconds into second half after Berahino was brought down by John Terry.
Berahino converted to take his league tally to 14 for the season and Chris Brunt lashed in a third goal on the hour for West Brom who are guaranteed to finish no lower than 13th place.
Loic Remy struck the post for Chelsea in the second half but the champions endured a miserable night in the Midlands. (Reporting by Justin Palmer; editing by Pritha Sarkar)