The striker has not played since undergoing knee surgery in June.
There had initially been hope the 32-year-old would be fit for the latter stages of the Champions League in August but that date passed and the target became the start of the new Premier League season.
However, Guardiola said that Aguero is still yet to return to training and will take time to get up to match fitness.
“We knew that the injury was difficult. In maybe one month or two months he will be ready,” said Guardiola.
“He has to come back on the pitch and regain the physical condition. Sergio was out for a long time and Sergio is not a guy who regains the physical condition quick.”
After missing out on the signing of Lionel Messi from Barcelona, Aguero’s absence leaves Guardiola with only one fit striker in Gabriel Jesus.
City open their Premier League campaign on Monday night looking to bounce back from a disappointing 2019/20 season as they finished 18 points adrift of Liverpool in second place.
