City will be without the Argentine on Tuesday against Marseille in the Champions League.
Aguero, who had just returned from injury, suffered a muscle problem in the Premier League draw at West Ham on Saturday.
“It will be 10-15 days minimum. If it is longer it will be three weeks to a month. I don’t know.”
Guardiola said he wants “Sergio to come back when he has fully recovered. We miss him a lot. What he has done for this club is extraordinary, he is a City legend.”
“He will be back as soon as possible.”
Pep Guardiola says Sergio Aguero could be out for a month.
— BBC Sport (@BBCSport) October 26, 2020
Brazilian Gabriel Jesus is also absent, but Guardiola said he did not regret not recruiting a striker in the transfer window.
“We couldn’t, the club does what it can, but we lacked income at the end of the season. And then we would need a striker of the level of Gabriel and Sergio, and we don’t have the means”.