Manchester City boss Manuel Pellegrini says it is "impossible to know" how long goalkeeper Joe Hart will be out for after he was injured in Wednesday's Champions League defeat at Juventus.
The 28-year-old was replaced by Argentine Willy Caballero, 34, in the closing stages of the 1-0 loss.
"He has a hamstring problem," said Pellegrini.
"We hope we don't lose him, but if for some reason he cannot play I have trust in Willy Caballero."
Mario Mandzukic scored the game's only goal at Juventus Stadium as the Italian side joined Manchester City in progressing to the last 16.
The defeat damaged City's chances of finishing top of Group D, with Juventus just needing to beat Sevilla, who have lost four of their five games so far, in their final group match to take first place.
"I think we were unlucky to lose," added Pellegrini. "They scored a goal which I think was a foul before Mandzukic finished but the referee didn't blow the whistle.
"However, we had chances and didn't score."