The midfielder has been ruled out for up to eight weeks after suffering a partial ligament tear in his left knee following a challenge from Mapou Yanga-Mbiwa in City's 2-0 win over Newcastle on Sunday.
The Frenchman has played a key part in City's good form, which has seen them go 10 matches unbeaten in the Premier League to be within a point of leaders Arsenal, and Pellegrini knows that Nasri will be missed.
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The City boss was speaking ahead of Wednesday's FA Cup third-round replay against Blackburn and warned that his team would need to be at their best in order to overcome their Championship opposition.
"[Blackburn] didn't surprise me. I think that they played a very good match at home," Pellegrini remarked. "I am absolutely sure that we must do a very good game if we want to win because they are very motivated, a strong team and they play exactly the way they want to play.
"We must be very attentive in defending and in counterattacks and defend the set-pieces. We need to try and score a goal and it will be very close."
Pellegrini talked up the importance of the FA Cup, declaring that he wants to win every trophy available to his team.
"It is very important," he noted. "We are for the moment the only team still in the four competitions and we will try and we want to win things."