The club says the midfielder's injury is not as bad as first feared and also say that Vincent Kompany and Sergio Aguero are fit to face Sunderland.
The Ivory Coast international was substituted after just 19 minutes of Sunday's 3-2 defeat to Liverpool at Anfield after picking up a muscle injury, but the City boss says the problem is not as bad as first feared.
"Maybe it’s not as bad as we thought at the beginning, but he needs at least ten days or two weeks to recover," Pellegrini told reporters.
Vincent Kompany and Sergio Aguero have also been struggling with injury, but City skipper has returned to training, and Pellegrini confirmed both could face Sunderland on Wednesday.
"Vincent [Kompany] is fine - he on the squad list for tomorrow and so is Sergio [Aguero] – we’ll see tomorrow who starts," he said.
Sunday's defeat means the Chilean's side is now seven points behind leaders Liverpool, but Pellegrini remains optimistic his side can still claim the title.
"We win our two postponed games and we're a point behind. If you don’t believe we can do it, you can’t believe in anything," he said. "It’s important we start with the same high performance we showed in the second half of the Liverpool game."