Aguero suffered a knee injury towards the end of last season, which ruled him out of the Premier League run-in and resumption of the Champions League knockout stage.
City manager Pep Guardiola has now revealed City's all-time leading goalscorer could be out of action for up to two months, in a huge blow to their hopes of reclaiming the title from rivals Liverpool.
“We knew that the injury was difficult," said Guardiola. “He's doing the job, maybe in one month or two months he will be ready.
“He has to come back on the pitch and regain the physical condition. Sergio was out for a long time and he's not a guy who regains the physical condition quickly.
"So I think in one or two months he will be ready."