Manchester City’s top scorer injured a hamstring during last week’s Champions League defeat to Barcelona and was already a doubt for the trip to Old Trafford.
The target now for Aguero is next Saturday’s visit to title rivals Arsenal, by which time he will have missed three games.
Speaking ahead of this weekend’s home game against Fulham, Pellegrini said: “We don’t know yet, but maybe next week he will start working with the squad.
“I don’t think the whole week, maybe at the end of the week he can be with the squad starting training. For United? No, no chance.”
The City boss, who has Stevan Jovetic fit again, gave a dismissive response when asked about a report linking Aguero with a summer move to Barcelona.
He said: “It is exactly the same as I respond to all the other links during the whole year. We cannot talk every week about the players that are coming here to Manchester City and the players that are going out.
“He is a player of Manchester City, he is very happy here and the club is very happy with him also.”
Of more concern to Pellegrini is the mystery knee injury which continues to keep out centre-half Matija Nastasic.
Nastasic has already missed eight matches, and the manager said: “He has at least a couple more weeks to recover.
“Of course we are concerned because the doctor doesn’t know what happened with him. He made all the exams that he needs but we don’t know what happened with him.
“We are taking other opinions. The doctor is in charge of that. The player feels a pain inside the knee but nobody knows what the pain is.”
Written by Steve Milne
