Maurizio Sarri admitted Chelsea’s second-half wobble against Slavia Prague highlighted a “big problem” which needs to be addressed.
The Blues led 4-1 at Stamford Bridge and 5-1 on aggregate after a one-sided first half of the Europa League quarter-final second leg.
But the visitors scored twice soon after the interval and Chelsea endured an anxious spell before confirming a place in the semi-finals, where they will face Eintracht Frankfurt.
“We started very badly in the second half. We need a solution,” said Chelsea head coach Sarri.
“We need to solve this problem, because in the last 10 matches we started like in this match, so it’s a big problem.
“Probably at the end of the first half we thought that we were qualified. But against these opponents, it wasn’t true.
“I have never seen something like that in the last 20 years. We did something wrong in the second half in the first 10-15 minutes.
“At half-time I told them that the target was to start with the same application and with the same attention, but we didn’t do it.
“But of course we are happy, because to get in a European semi-final is not very easy.”
Asked why the problem had occurred, Sarri said: “I don’t know. It’s not easy to understand, otherwise we could solve the problem very easily. It’s very difficult to understand why.
“Probably at the end of the first half, when we have lowered the application, we are not able to restart. But it’s a problem.”
Sarri started with Eden Hazard, who went off during the second half after picking up a knock. The Belgian is expected to be fit for Monday’s home game against Burnley.
“Nothing serious I hope, and I think. It’s only a knock,” Sarri said.
“It’s a strong knock, but I think we can recover him for the next match on Monday.”