Chelsea boss Frank Lampard admits Ross Barkley's injury "looks a bit nasty" but he is waiting for a doctor's assessment on Sunday.
Barkley was forced off after 43 minutes of Saturday's 1-0 Premier League defeat of Newcastle United, hobbling off the pitch with an ankle problem to be replaced by Mateo Kovacic.
The England international, who started in place of the injured N'Golo Kante, is now expected to have tests ahead of Wednesday's Champions League trip to face Ajax.
Cesar Azpilicueta also suffered a knock in the closing stages, but Lampard is hopeful the defender has not sustained any serious damage.
Speaking after the match, which was settled by Marcos Alonso's 73rd-minute strike, Lampard told reporters: "I think Azpi was alright, he didn't say anything when he came off.
"Ross turned his ankle. It looks a bit nasty at first glance, but the doctor wants to have a look at it tomorrow. I have no more on that."
When asked if he could be fit to face Ajax, Lampard replied: "I genuinely don't know. We will assess tomorrow."
Chelsea's win, their fifth in a row in all competitions, means they are fourth in the Premier League after nine matches.