Enzo Maresca warned Chelsea to stop "giving away presents" after their latest red card played a key role in a shock 3-1 defeat by Brighton at Stamford Bridge on Saturday.
Maresca's side were leading through Enzo Fernandez's 24th-minute header when Blues defender Trevoh Chalobah was dismissed for a poor tackle on Diego Gomez.
Chalobah's 53rd-minute red card for denying a goalscoring opportunity put Chelsea in an awkward position and they collapsed in the final stages as Brighton struck three times.
Danny Welbeck equalised in the 77th minute, and Maxim De Cuyper's header put Brighton in front in stoppage time.
The substitute was left unmarked to nod the ball home after Mats Wieffer had turned Yasin Ayari's cross back into the area.
Welbeck added the third goal in the final seconds to condemn Chelsea to a second successive Premier League loss.
Last weekend, Chelsea goalkeeper Robert Sanchez was sent off after four minutes of a 2-1 defeat at Manchester United.
For the second time in seven days, Maresca was left searching in vain for answers to Chelsea's habit of gifting opponents the advantage with their lack of discipline and poor defending.
"The message (in the dressing room) was clear," said Maresca. "Against any team in the Premier League, you cannot continue to give away presents.
"We are giving away presents. The red card is a big mistake because the game completely changed."
Chelsea had started well and took the lead when Moises Caicedo slipped the ball into Reece James, whose cross was nodded in at the far post by Fernandez.
Chalobah's red card changed the momentum after a poor pass from Andrey Santos prompted the centre-back to make a crude challenge on Gomez in a bid to stop him advancing on goal.
Chelsea slipped to eighth place and were booed off at the final whistle, with Premier League leaders Liverpool next up for the spluttering Blues on 4 October.
"It's a tough one because we were in control of the game," Maresca said.
"We scored one, we could have scored more. We had so many shots first half, 70 per cent of possession - everything was fine.
"Then suddenly because of our mistake, with the red card the plan completely changed. We played two games - first half, then unfortunately, after the red card.
"For sure you cannot continue to make mistakes. They are big mistakes, game-changing, red card against United and today."