N’Golo Kante will miss Tuesday’s Champions League group game against AC Milan after suffering a reaction to an existing hamstring injury.
The 31-year-old France international has not played since mid-August due to the hamstring issue.
He returned to training a week ago but suffered the reaction on Monday.
“N’Golo, we are just waiting — he has had a reaction in training so we are just waiting on the news from that,” said coach Graham Potter.
“It’s certainly not ideal and it’s disappointing for him and for us. We have to wait and see the extent of it and go from there.”
Morocco winger Hakim Ziyech will also be missing the Blues’ visit to the San Siro through illness.
Red Bull Salzburg currently lead Group E on five points, with Chelsea and Milan, who they beat 3-0 in the reverse fixture in second and third on four points.