Branislav Ivanovic's return to Chelsea lasted less than 15 minutes as the West Brom defender came off the bench and went off injured in the first half.
The 37-year-old returned to the Premier League after joining West Brom from Zenit last September.
The centre-back was named among the substitutes as he returned to Stamford Bridge for the first time since he left the Blues in 2017, but his afternoon did not go as planned.
What happened?
Ivanovic was brought on to replace Dara O'Shea just 24 minutes into the match after the latter was knocked down by a collision on the sideline.
After Ivanovic came on, Chelsea took the lead through a Christian Pulisic goal and saw centre-back Thiago Silva sent off with a second yellow card.
But 13 minutes after he was brought on, Ivanovic pulled up and had to be replaced after stopping a Timo Werner run.
Ivanovic was replaced by midfielder Callum Robinson.
West Brom fight back
West Brom managed to turn the scoreline around just before half-time, as Matheus Pereira got on to a long pass and managed to beat Edouard Mendy.
The Brazilian followed up shortly afterwards with another strike to give his side the advantage.
Ivanovic's replacement went on to extend the visitors' lead shortly after the hour mark after Darnell Furlong cut back to him.
Five minutes later, West Brom were 4-2 up as Mbaye Diagne smashed into the net.
Mason Mount pulled one back for Chelsea shortly afterwards but Robinson struck again to seal a 5-2 victory for his side.
The clash in London is a must-win for West Brom, who are second bottom of the Premier League and now seven points from safety.
Chelsea, meanwhile, are in fourth place and looking to hold on to the final place that guarantees a spot in next season's Champions League group stage.
