Nasri has been ruled out for up to eight weeks after suffering a partial ligament tear in his left knee following a shocking 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 English Premier League to be within a point of leaders Arsenal.
"He is a very important player for us. He was also playing in a very good moment so we hope he can come back as soon as possible," the Chilean said.
"But I think that less than six or seven weeks would be difficult."
Pellegrini's team were held to a 1-1 draw by Championship outfit Blackburn to force the replay at Etihad Stadium.
"(Blackburn) didn't surprise me. I think that they played a very good match at home," Pellegrini said.
"I am absolutely sure that tomorrow 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 tomorrow in defending and in counter-attacks and defend the set-pieces. We need to try and score a goal and it will be very close."
"It is very important. It is very important not only because it is an important game but we have a lot of games also," he said.
"We are for the moment the only team still in the four competitions and we will try and we want to win things."
