The French international, who has only been used sparingly by Pellegrini so fa
"Nasri had a problem with his hamstring," Pellegrini has publicly explained in the wake of the weekend.
Asked by the media about a possible timeframe for the player's recovery, he added, "I don't know because I didn't talk with the doctor yet, but I think it will be very difficult for him (for Sevilla)."
Having picked up his problem after City had made all three substitutions, Nasri stayed on the field when he might otherwise have been taken off as a precaution.
"Unfortunately we knew that he was feeling something after we made the third change so if not maybe I can change him before," Pellegrini explained. "He didn't run too much, he stayed in the middle so we'll see in the next few days what happens with him."
City, who are already missing David Silva and Sergio Aguero, go into the home fixture with Sevilla in need of a win to bolster their hopes of reaching the knockout rounds of the Champions League for the third consecutive year.
The Sky Blues were defeated by Juventus at the Etihad in the opening game and were forced to come from behind to beat Borussia Monchengladbach in Germany in their last European outing.
The home game against the current Europa League holders will be followed by the return game in Spain, with a tough trip to Turin also still to come. Immediately after this week's clash with Sevilla, there is also the Manchester derby to think about for Pellegrini and City.