N’Golo Kante has suffered a setback in his recovery from a hamstring injury.
Kante, out since limping off against Tottenham on 14 August, recently returned to training.
But he missed Monday’s session and did not travel with the squad to Italy for the Champions League game against AC Milan.
Chelsea are also without Hakim Ziyech, who is ill, and the injured Wesley Fofana.
Boss Graham Potter confirmed on Monday evening that there is new concern over Kante, who has been plagued by injuries in recent seasons.
The Frenchman has had an adverse reaction following his recent return to training and it is not yet clear when he might be available again.
Potter said: “It’s not ideal and is disappointing for both him and us. We need to see the extent and then go from there.”