The France international has not featured since City's 2-0 win over Newcastle United in January, and Pellegrini's team have slipped to third in the Premier League after taking just one point from their last two games against Chelsea and Norwich City.
And with Pellegrini aiming for a measure of revenge over Jose Mourinho's side in the FA Cup fifth round, he admitted his team has missed the influence of Nasri.
"He is a very important player, it is impossible not to miss him," the Chilean said on Friday.
"But we also had other injured players before him - David Silva, Sergio Aguero, Stevan Jovetic and Vincent Kompany - and we always managed to continue winning.
"In the last two weeks I said this was not an excuse because we have had a lot of injuries during the year.
"In the last two weeks maybe we have had injuries in the same position and that has been a major problem.
"Samir Nasri is a very important player and we always miss him, but with the squad we have I think we can also win without him."
Javi Garcia is also fit again for Pellegrini, although Matija Nastasic, Fernandinho and Sergio Aguero are yet to return from injury.
The former Real Madrid and Malaga boss went on to say he is focused on picking the right side for the FA Cup tie, with Barcelona visiting the Etihad Stadium in the UEFA Champions League on Tuesday.
"I am not thinking about the quality of the opposition. I always think the opposition is always a quality team and a difficult team," he continued.
"We will see tomorrow what is the best 11 starters to play against Chelsea. Also we have an important game on Tuesday and we will see tomorrow the best team to start."
