The Blues won 4-3 at Stamford Bridge, going through 5-3 on aggregate in a contest that saw Sarri’s men race to a 3-0 lead after just 15 minutes.
But Chelsea have developed a nasty habit of conceding early in the second half and once again there were some hairy moments as the visitors hit back twice to trail by just one on the night.
“We played very well in the first half,” said Blues boss Sarri. “But as usual in the last two months we started very badly in the second half.
“We need a solution. In the last 10 matches we started like in this match, it’s a big problem.
“Probably at the end of the first half we thought we had qualified, but against this opponent it was not true. I have never seen something like these opponents physically, so it was clear the second half could be difficult for us.
“But we did something wrong in the first 10 minutes of the second half.
“At half-time I told them to start with the same application as they started the first, but we didn’t do it. It’s not easy to understand.
“I could try without going in the dressing room at half-time if I could solve the problem.”
Pedro opened the scoring following a flowing move involving Cesar Azpilicueta and Olivier Giroud, with the Spaniard applying the finish.
Pedro should have claimed the second but he somehow side-footed Eden Hazard’s inviting ball across goal on to the post.
Fortunately for the Blues the ball ricocheted into luckless defender Simon Deli and bounced into the net.
N’Golo Kante then set Pedro through to tee up Giroud for his 10th goal in the competition this season.
That should have been that, but Chelsea’s defence dozed off in the 26th minute to allow the unmarked Tomas Soucek to pull one back with a header at the far post.