City have stuttered in the Premier League, losing to title rivals Chelsea 1-0 before a goalless draw at Norwich prevented them from capitalising on favourable results elsewhere.
Pellegrini's side have lacked a cutting edge, despite a wealth of attacking options, and the Chilean conceded that Nasri has been sorely missed during a five-week absence with knee ligament damage.
The Frenchman is back in contention for this weekend's FA Cup fifth-round tie with Chelsea, but Pellegrini is careful not to put too much pressure on his shoulders.
"He is a very important player, it is impossible not to miss him," he said.
"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."
Top scorer Aguero and midfielder Fernandinho remain on the sidelines, but Javi Garcia is also fit again after a muscle strain.
"I think it is very important to have players, Samir Nasri and Javi Garcia, fit for this game. It is very important for us," said Pellegrini.
"(Matija) Nastasic, Fernandinho and Aguero are not fit for this."
City can swiftly avenge the home defeat by Chelsea after Pellegrini appeared to lose the tactical battle with Blues boss Jose Mourinho in the league.
But Pellegrini will not alter his approach for the cup clash at the Etihad.
"I think it will be a similar game. We have a style, we won't change it," he said.
"We know how they play, their best players, we made mistakes, hopefully we will put them right. But I won't change the way we play, our style, just because it's Chelsea."