The influential midfielder limped out of the 3-2 defeat Liverpool 12 days ago with a thigh injury and has not featured since, missing games against Sunderland and West Brom.
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"We have [David] Silva, Jesus Navas and [Matija] Nastasic out with injury - all of the other players are fit," he said.
"David is working, improving. With Navas' ankle, it's the same. These three players are injured - we don't know how much longer for David.
"It's good to have Sergio Aguero back. He's a top player and makes an important difference. We played a lot of games without him but always he's very important for the team."
City will have one eye on events at Anfield before their game on Sunday, with title rivals Liverpool and Chelsea going head-to-head.
With Chelsea taking on Atletico Madrid in the second leg of their Champions League semi-final next Wednesday, Jose Mourinho has hinted he may field a weakened side, but Pellegrini knows that is out of his hands.
"I don't know what Chelsea are going to do," he said. "The only important thing and the only thing we can do is to win our game and that is what we're going to do from now until the end of the season.
"I think that every team tries to do the best thing for their teams. We must win our games."