Maurizio Sarri is bemused by Chelsea's recent problems in the second half of matches after they ended up only scraping a 4-3 win over Slavia Prague in the UEFA Europa League on Friday (AEST).
Chelsea appeared to be in complete control at half-time at Stamford Bridge, as they had cruised into a 4-1 lead.
But a fine Petr Sevcik brace early in the second period had Slavia on the up and pushing for further goals, as they still needed two more to secure progress to the semi-finals.
Ultimately Chelsea did enough and went through 5-3 aggregate victors, but Sarri was again left baffled by the Blues' second-half woes.
Chelsea's 2-0 defeats to Liverpool and Everton in the Premier League over the last five weeks had all of the goals conceded in the second half, and Sarri is concerned.
"[Chelsea were] very good in the first half, I think. Really, [we played] very well," Sarri told reporters at his post-match news conference.
"As usual in the last two months, we started very badly in the second half. We need a solution. We need to solve this problem.
"In the last 10 matches, we started like in this match [in the second half]. It's a big problem. We have to concede it's a match for qualification, not for three points, but probably at the end of the first half we thought we were qualified.
"But against these opponents, it's not true. They have physical numbers. I have never seen something like that [Slavia's fitness] in the last 20 years.
"The number of accelerations, the metres in high speed, the running... I've never seen anything like that in these opponents.
"So, the second half clearly could be difficult for us, but we did something wrong. In the first 10 or 15 minutes of that half, they scored from 18 metres without being opposed, twice.
"At half-time I told them that the target was to start with the same application and attention in the second half, but we didn't do [it].
"I don't know [why], and it's not easy to understand, otherwise we can solve the problem very easily.
"It's very difficult to understand why. I can try without going into the dressing room at half-time. I can try if that'll solve the problem."
Chelsea will face Eintracht Frankfurt in the semi-finals, after they beat Benfica 2-0 at home to secure progress on away goals following a 4-4 aggregate draw.