City's playmaker damaged a thigh muscle while away on international duty with Spain and, next Sunday's derby game against Manchester United is a more realistic target than the midweek trip to Plzen in the Czech Republic.
City manager Manuel Pellegrini said: "We will see next week what happened with him. We hope he can be, not sure - we will try for the derby."
Captain Vincent Kompany (groin) is rested this weekend but should travel to Europe as City begin their attempt to get out of the competition's group stage for the first time.
Pellegrini said: "Kompany is okay but I think it is a big risk for him to play three matches in one week. He needs two or three more days to be 100%."
City's game at Plzen is live on Sky Sports 2 from 7pm this Tuesday, and you can also watch the Manchester derby live from 3.30pm on September 22 on Sky Sports 1