Thomas Tuchel criticised the attitude of Chelsea’s players after their 3-3 draw with Zenit in the Champions League.
The hosts’ stoppage-time equaliser cost Chelsea top spot in the group and means they go into the knockout stage unseeded.
Timo Werner gave them the lead after just 84 seconds but they were behind by half-time.
Romelu Lukaku equalised and Werner put them 3-2 up, but they were unable to see the game out.
Speaking at his post-match press conference, Blues boss Tuchel admitted: “We had a very good first 15 minutes and then we stopped doing the things we did in the first 15 minutes.
“In my opinion we had a feeling that ‘This is OK. We are in charge. We are the better team’.
“But we were forgetting why we were the better team – (it was) because of the investment, the level of concentration. The level was high enough to be the better team.
“And once this drops 5%, 10% – and it did – once we change our behavior we get punished at the moment. It happened at West Ham and again today.
“We started playing balls back, not attacking with the same aggression and same hunger and we’ve been punished twice.”
Chelsea were poor at the back and both Zenit goals came after defensive lapses.
“My analysis is very clear: our behaviour changes when we have a lead,” said Tuchel.
“This is something we never did and should never do. If you change then you allow the chance to be punished. The challenge is to close the door.”