The defeat put an end to the unbeaten run of New Manager Thomas Tuchel who took over from club legend Frank Lampard.
Chelsea scored first through Pulisic in the 27th minute.
But things went bad when Thiago Silva received a second yellow card of the afternoon which led to his dismissal.
West Brom grew into the game with goals from Costa Pereira (45’+2minutes, 45’+4minutes), Robinson made it 3-1 on the 63’minute, Diagne scored on the 68’minutes to make it 4-1 before Mount reduced the deficit in the 71st minute. Robinson wrapped up the match with his second goal of the afternoon leaving the score line at 5-2 full-time.
Semi Ajayi displayed a decent defensive performance, the Nigerian defender recently helped the Super Eagles qualify for the 2022 Africa Cup of Nations and was immediately handed a starting role in the Baggies’ first game after the international break.
The big centre-back justified his inclusion of himself in the encounter.
Ajayi played full 90 minutes in the game to mark his 25th Premier League appearance of the season, he made three tackles, two blocks, two clearances, three interceptions and had a 100 percent successful pass rate.
The Nigeria internationals will hope to continue his impressive performances in the next fixtures when West Brom battled against Southampton in the league game on April 12.