Reece James will see a specialist this weekend to help determine the extent of his knee injury.
It comes amid worrying reports that the problem is more serious than initially thought and that the England international is in danger of missing the World Cup.
Blues boss Graham Potter was reluctant to be drawn on the issue when asked about it at a news conference on Friday afternoon.
“He’s due to see a specialist at the weekend. There’s not much I can add. The rest would be speculation,” said Potter.
“We’ll wait and see. There’s no point in looking into the doomsday scenario yet.”
James suffered the injury during this week’s Champions League victory over AC Milan at the San Siro.
Potter added that N’Golo Kante will also be assessed this weekend and that Wesley Fofana is unlikely to play before the World Cup.
Fofana has been sidelined by a knee injury and fellow France international Kante has suffered a setback in his recovery from a hamstring problem.
The injury-plagued Kante has been out since going off against Tottenham in mid-August and is expected to remain sidelined for a number of weeks.
“He is due to see a consultant at the weekend,” Potter explained.
“It’s a setback. It’s not good news. But again I can’t give you any more.”