City go to Aston Villa in the Premier League today, having taken just a point from two away games so far this season.
Pellegrini said: "In this moment, Nasri is playing very, very well and I also talk often with him because I know how much he can do as a big player.
"His performance I am evaluating and he knows that he must play the way that I know he can do it. If he doesn't do it, well then maybe we have another choice.
"Every team that wants to win something must demonstrate every week the intensity, quality and spirit that we have in this team."
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