Federico Chiesa scored the only goal of the game just after the restart with a thunderous finish, Chelsea were largely subdued in the game with only one shot on target.
“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.
“We were so good yesterday in training, but not good enough, not free enough today. I don’t know why, it was the whole team,” he added.
They were caught out after the restart as Chiesa scored the earliest Champions League second-half goal since at least 2003-04.
“At this level you cannot concede an easy goal like this in the first seconds of the second half,” said Tuchel.
“We should have been much, much sharper. We should have asked more questions.”
Chelsea will hope to bounce back against Southampton in the Premier League on Saturday.