A stunned Thomas Tuchel delayed his dressing-room debrief until Sunday after Chelsea deteriorated from a well-oiled machine to a "rusty" heap in a 5-2 thumping by West Brom.
The head coach had led Chelsea on a 14-game unbeaten run after succeeding Frank Lampard in January, but they were overrun at Stamford Bridge by Sam Allardyce's relegation battlers.
Christian Pulisic had given a slow-starting Chelsea the lead after 27 minutes, but a red card for Thiago Silva moments later led to the hosts going limp, and by the time their second goal arrived from Mason Mount, the Blues had conceded four times.
Matheus Pereira and substitute Callum Robinson both scored twice in the unexpected rout, with Mbaye Diagne also netting for the second-bottom Baggies.
Robinson became the first player in Premier League history to score at least twice in home and away games against Chelsea in a single season.
Chelsea have a UEFA Champions League quarter-final first leg against Porto coming up in Seville on Thursday (AEST), and Tuchel is determined for any personal response to his first defeat to be a measured one.
"We cannot lose our heads and we cannot take away the trust we have with these players," Tuchel said.
"It was the moment after the game to be honest but to be calm, and to breathe and tell everyone to breathe. It will be a hard afternoon and evening for everyone.
"We have to accept it now. It's our first loss together and it's important to find a way to deal with it together. There were some quick words to calm everyone down and delay the talks until tomorrow. Because now is too emotional, now is too much frustration, and this is not productive enough."
Tuchel felt Chelsea were off their game even from an early stage, complaining they missed "a lot of opportunities to finish this game" before West Brom began their hot streak.
They had 18 shots to West Brom's 14, but Chelsea would have been flattered by anything but defeat.
"We were not able to adapt to one man down," Tuchel said. "There is no need to concede five goals if you are one man down, with our quality.
"But our defending was clearly not on the level it used to be. It's hard to accept but there's no other solution than to accept it and move on forward."
As Allardyce savoured becoming the first manager to win away at Stamford Bridge in the Premier League with three different clubs (Bolton Wanderers, Crystal Palace and West Brom), Tuchel was left to consider what went so badly wrong.
The German coach was asked whether the result was a "wake-up call", but he suggested he had not considered his team in need of such a jolt.
"If we look at this game in four weeks hopefully we call it a wake-up call because that would mean we really woke up and started a new series [of unbeaten games]," he said.
"For me, it was not necessary. I did not feel an overconfidence, I didn't feel we were arrogant or without the effort. They started to defend very deep. We had high ball possession, we created good moments and had good ball recoveries up the pitch. But still at 11 against 11, we had crucial ball losses in our own half, which produced two set-pieces right around the box.
"I was a bit more concerned by our body positioning, our positioning for the build-up and everything seemed a bit rusty and more rusty than in training.
"And that's why I was surprised [by the early stages of the game]. But I could accept it, I did not have the highest expectations. I thought if it takes us a while to come into this match, I will accept it.
"But to give a red card away like this straight after a goal, maybe we did not feel the risk. I will talk to the players about it. But the attitude in defending was clearly not the same like we did before."