Guardiola said this was because Aguero was yet to train with the club since suffering a knee injury in June.
Guardiola said the 32-year-old may not be fit until November.
“We knew the injury was difficult,” Guardiola said.
“He still hasn’t had one training session with us and Sergio is not a guy who regains his physical condition quickly.
“He has been out for a long time. So, maybe in one or two months he will be ready.”