Thomas Tuchel said Chelsea “stopped defending” as Brentford ran riot in a sensational 4-1 derby victory at Stamford Bridge.
Brentford stunned the European champions on Saturday, emphatically securing a first win over their west London rivals since 1939.
Antonio Rudiger’s long-range thunderbolt put the Blues in front early in the second half, but Thomas Frank’s side hit back with three goals in the space of 10 minutes.
Vitaly Janelt struck twice either side of Christian Eriksen’s first goal for Brentford and Yoane Wissa added a fourth in the closing stages just after coming on.
Chelsea had won five consecutive games since Roman Abramovich put the club up for sale, but they were picked off time and again as rampant Brentford moved 11 points clear of the Premier League relegation zone.
Blues head coach Tuchel felt his side’s performance was out of character ahead of a Champions League quarter-final first leg against Real Madrid on Wednesday.
He told BBC Sport: “Nobody saw it coming, especially after going 1-0 ahead. It was very untypical of us. But we stopped defending. We were not aware of enough of the danger, were sloppy with the defending and got punished.
“They made the most of it in those 10 minutes. Right after their third goal we had 10 minutes where we had big, big chances and we missed them.
“They were lucky and clinical enough to get three goals. We did not defend like it is needed to in the Premier League and got punished.”
Tuchel had expected third-placed Chelsea to be vulnerable in their first game back after the international break.
He explained: “The start was not a surprise because we came from all over the world from national duty so I expected it to take time. The first 25 minutes we looked a bit leggy and needed to find rhythm.
“The opponent defended deep with 10 men behind the ball, so you need a shot or action to open the game, which we did. Then we gave the game away very cheaply.”
Chelsea will now regroup before doing battle with LaLiga leaders Madrid at Stamford Bridge in midweek.
“It is not about Real Madrid. It is about us. It is one day off now and on Monday we start again,” he said.