Manchester City goalkeeper Ederson is an injury doubt to face title rivals Liverpool after he was withdrawn at half-time in their draw with Atalanta on Wednesday.
City travel to Premier League leaders Liverpool on Sunday (16:30 GMT).
"It's a muscular problem," manager Pep Guardiola said. "It was a risk so we took him off as he was feeling it late in the first half."
Asked whether he will be fit for Sunday, Guardiola said: "I don't know".
The Brazilian was replaced by Claudio Bravo at the interval in Milan, before Kyle Walker was forced in goal after Bravo was sent off for a sliding tackle on Josip Ilicic outside the box.
Ederson played every game in City's successful title defence last term, keeping 20 clean sheets in the Premier League, the second highest behind Liverpool's Alisson in the top flight.
He has also started every league and European game for City so far this season as Guardiola looks to defend his domestic title and claim the first Champions League trophy for the club.
City will be aiming to reduce Liverpool's six-point lead at the top of the table this weekend.