Maurizio Sarri has taken the blame for N'Golo Kante's hamstring injury that has ruled out the Chelsea midfielder for the next two games.
The Frenchman limped off inside 10 minutes of Chelsea's 3-0 win over Watford.
Sarri confirmed the injury was a hamstring problem that will keep out the key midfielder for Chelsea's important games against Eintracht Frankfurt on Thursday in the Europa League and Leicester City on Sunday.
He admitted it was a mistake starting him just three days after their 1-1 draw in the first-leg in Frankfurt.
"He has a problem with his hamstring, but I think I made a mistake, he should have rested today," he said.
"Sometimes it's difficult to put someone on the bench so important like Eden Hazard or Kante - it's a risk.
"We will wait for the morning, it's very difficult to recover for the next two matches. We have to recover him for the final."
Chelsea's slow start...
Chelsea looked very lethargic in the first-half against Watford and were lucky to still be in the game at half-time.
Sarri put the poor performance down to tiredness following their short turnaround in the Europa League.
"I think in the first half we were tired, mentally. We were in trouble. Then they lowered their intensity. We were lucky to score after two minutes - we improved mentally after first goal. We played very well for 35 minutes then. It's not easy to play after 60 hours. After Europa League away games we've only won one match.
"When you are not brilliant physically you have to move the ball, the ball isn't tired, the man is tried. In the second half we did it better as something changed in our minds."