The hosts then conceded two quick-fire goals in first-half stoppage time courtesy of Matheus Pereira to hand West Brom a half-time advantage.
Sam Allardyce’s side continued to trouble Chelsea’s usually watertight defence throughout the second half, with Callum Robinson and Mbaye Diagne firing in to stretch their lead, before Mason Mount scored with 19 minutes remaining to give the Blues hope of snatching a point.
However, the result was put beyond all doubt when Robinson chipped Edouard Mendy in the dying seconds for his second goal of the match and West Brom’s fifth.
Chelsea’s defeat ended their 14-match unbeaten run under Tuchel and curbed an impressive spell of seven games without conceding a goal.
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“Nothing so much until the red card, after that we were not able to cope with the situation,” said Tuchel.
“I don’t know why. After the red card, we could not adapt. We gave away two easy goals. Even in the second half, what could go wrong went wrong.
“We have to accept a big loss today. Hopefully it’s a big wake up call for all of us – me included.”
“I didn’t see,” he added. “Twice, it was not his mistakes. We made big mistakes before the two yellow cards. We are responsible.
“Too many easy ball losses. Then we got a red card and it was impossible for us to adapt.”
Chelsea have been imperious in defence since Tuchel’s arrival, with the German manager’s preferred three-at-the-back system helping to shore things up, but the Blues were at sixes and sevens throughout Saturday’s encounter.
The 47-year-old suggested that a lot of work will be done on the training ground between now and Wednesday’s Champions League quarter-final first-leg clash against Porto, claiming that a significant response is required.
“Today we conceded twice and it changed the momentum. It calmed them today. Everything that could go wrong, went wrong.
“Every shot was a goal for them. We had big chances – but no. It was clearly not our day.
“We will get the right response because this is sport. Today we digest it because it’s a lot. A lot to digest. I did not see it coming.
“OK, now we have to take our responsibility and shake it off. [We will] continue tomorrow.”