Cape Town - Pep Guardiola is uncertain as to the extent of the injury sustained by Kevin De Bruyne on Saturday ahead of his club assessment.
The 25-year-old Belgian, who has been one of the Premier League's stand-out performers so far this campaign, pulled up in the 81st minute of City's 3-1 win over Swansea.
He was replaced by Jesus Navas before heading down the tunnel for treatment.
After the game, Guardiola said he felt the injury was "muscular".
"It looks like it was muscular, but I am not a doctor," Guardiola said. "It is a pity because he is an important player.
"That's why we have to have a big squad as everyone will help us."
De Bruyne will be assessed by the club doctor on Sunday with the player naturally hoping his injury is not serious.
City play Celtic in the Champions League on Wednesday ahead of a Premier League clash with Tottenham Hotspur next weekend.