Aguero, City's top scorer, has returned to training after missing four games with a hamstring injury, but is not yet ready to play in Saturday's clash with Southampton at the Etihad.
Pellegrini had previously suggested the 25-year-old could feature this weekend, but will let his star striker off the leash against Premier League leaders Liverpool the following Sunday.
"Sergio is okay. He has worked just now with the squad but he just started on Thursday, so he needs more work to be 100 per cent fit," said Pellegrini.
"He is not in the squad but he doesn't have any problems with his injury.
"For this week it is too soon for him to start playing because he just worked two days, but next week against Liverpool he will not have any problems."
Reports suggest City have opened contract talks with French midfielder Samir Nasri, but Pellegrini insists he is only concerned with his side's league campaign.
"We are just thinking about this match we have to play against Southampton," said the Chilean.
"All the other things - you are always talking and thinking and working - but in this moment it is not the most important thing."
City are sitting in third place in the table, four points adrift of Liverpool, but with two games in hand.
Pellegrini believes the fiercely contested title race might not be decided until the closing weekend of the campaign.
"Yes I think so. It will be very close until the ending," he said.