"[We played] very well in the first half I think. As usual, we started badly in the second half. We need a solution. In the last ten matches, we started like this match. It is a big problem.
"We had to get qualification and maybe we thought we had at the end of the first half I have never seen something like that, like these opponents.
"It was clear the second half would be difficult. We did something wrong. They scored with a kick with no one near. We are happy to go through."
Chelsea were impressive in the first half at Stamford Bridge. The hosts scored four times to move into a 5-1 aggregate lead over the Czech opponents.
ESPN FC's Liam Twomey highlighted their dominance:
However, it was a different story entirely after the break. Two goals from Petr Sevcik made it 4-3 on the night and offered the visitors hope they could complete the comeback.
Simon Johnson at the Evening Standard summed up Chelsea's performance after the break:
This is not the first time this season that Chelsea have flopped in the second half, and Sarri has been criticised for his team talks:
Meanwhile, former Chelsea manager Glen Hoddle says the team are lacking leadership on the pitch:
Football on BT Sport @btsportfootball
"They need a leader out there. John Terry playing in that team tonight, that wouldn't have happened in the second-half." "They don't have many leaders there...." Glenn Hoddle thinks Chelsea need someone on the pitch to keep the side focused throughout the 90 minutes. https://t.co/fxfXyDSJFD
Captain Cesar Azpilicueta told BT Sport (Joe Krishnan at the Evening Standard) after the match that his team must learn from their errors.
"I think it's our mistake and one we are making very often," he said. "The result is not easy and we keep conceding early goals and making the same mistakes in the second half."
Chelsea's second-half struggles will concern Sarri, and he will need to improve his side if they are to lift the Europa League this season.