Chelsea were cruising at half-time with a four-goal aggregate lead before Petr Sevcik produced two sensational strikes as the nerves crept into a previously dominant performance at Stamford Bridge.
The Blues also conceded early in the second half of their loss to Everton last month, as well in their victory against Cardiff on 31 March, while just last weekend Liverpool hit them with two goals in eight minutes at the start of the second period as they fell to a Premier League defeat at Anfield.
"We played very well in the first half, but as usual in the last two months, we started very badly in the second half," said Sarri.
"Probably at the end of the first half we thought we had qualified and against this opponent it was not true.
"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.
"We need a solution but I'm really really happy because we played well in the first 45 minutes and we are in a European semi-final which is very important to us."
Having secured a valuable 1-0 advantage in the Czech Republic, Chelsea raced into a three-goal lead in west London courtesy of goals from Pedro, Olivier Giroud and a calamitous Simon Deli own goal.
Slavia captain Tomas Soucek headed in his side's first goal before Pedro doubled his tally just 63 seconds later.
Sevcik's double changed the mood and, buoyed by their noisy supporters, the visitors pushed forward as they tried to force a way back into the game before time defeated them.
Chelsea travel to Germany for the first leg of their last-four tie with Eintracht Frankfurt on Thursday, 2 May.[credit: BBC]