Reece James suffered a fresh injury setback when he was forced off in the first half against Everton Sunday.
The Chelsea captain looked dejected as he had to be replaced after just 27 minutes at Goodison Park.
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James has only recently returned from a hamstring injury and Chelsea will fear the right-back will now be set for another spell on the sidelines.
James was making just his fifth Premier League start of the season against Everton.
He did not receive any treatment on the pitch but had to be replaced by Levi Colwill on Merseyside.
Chelsea were without James for more than two months after he injured his hamstring against Liverpool on the opening day of the season.
The England international was eased back into action with substitute appearances before he started and helped Chelsea get impressive results against Tottenham and Manchester City.
But James was sent off in the 4-1 defeat at Newcastle and his season look set to be further disputed.
His fitness issues are becoming a major concern for Chelsea.
Chelsea did not initially give any indication of the nature of the injury suffered by James.
“The Blues are forced into a change. Colwill is on for Reece James,” the club said on X. “Conor Gallagher takes the armband.”