Thomas Tuchel said "I don't know why" Chelsea did not play well in their 1-0 Champions League defeat by Juventus.
Federico Chiesa scored Juve's goal 10 seconds into the second half.
The Blues have now lost twice in five days - with only one shot on target in total - after a 1-0 home Premier League defeat to Manchester City on Saturday.
"I felt we were a bit slow and tired, mentally slow in our decision making. That's why it's a strange one to analyse," boss Tuchel told BT Sport.
This was only the second time European champions Chelsea have lost consecutive games since Tuchel was appointed in January.