Vincent Kompany has suffered a groin injury but the seriousness of the problem is still being assessed by Manchester City's medical staff.
The City skipper left the pitch two minutes into injury time of the EFL Cup victory over Swansea City on Wednesday night in his first game in four months.
The defender left the pitch after stretching to try to stop Gylfi Sigurdsson's consolation goal and punched the ground in frustration as he stood up.
A statement released by City read: "No date has yet been set for his return. The positive for Kompany and City is that the injury is not a recurrence of the calf problems that disrupted his progress last season."
Kompany has remained in South Wales with the first-team squad, who have stayed close to the city as they play Swansea again in the Premier League on Saturday.
He will not feature at the weekend and will have further tests to see how serious the problem is.
Kompany was making his first appearance since suffering a thigh strain in the Champions League semifinal against Real Madrid that ruled him out of Euro 2016.
He has been dogged by muscle injuries in recent seasons suffering five separate problems last seasons, which restricted him to just 20 starts from 59 games. He also played only 31 games of the 51 matches in 2014-15.
City have been patient not to rush Kompany back and have been giving him blood tests to monitor his progress.
Guardiola said after the match "hopefully it will not be a big, big problem."