Chelsea boss Maurizio Sarri admits Ruben Loftus-Cheek has been struggling with a back injury this season.
The England international shook off his fitness troubles to put an outstanding display in Thursday's 1-1 draw with Eintracht Frankfurt in the Europa League semi-final first leg.
Speaking after the match, Sarri said: "He is a very important player. He has improved a lot during the season.
"When he is at the top physically he is a very important player for us.
"It's not easy for him to train with consistency because of his back, but in this match he was really fit.
"Unfortunately with ten minutes to go he had cramp and asked for a substitution."