The France international midfielder has been in good form in 2013-14 but a ligament tear in his knee is set to keep him out for up to eight weeks, disappointing the Chilean coach
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.
"He is a very important player for us," the Chilean told the press. "He was also playing in a very good moment so we hope he can come back as soon as possible but I think that less than six or seven weeks would be difficult."
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."