Manuel Pellegrini: "I am not a doctor. It is very difficult.
"The doctor has said he has at least one month or eight weeks. We will see.
"I don't how many weeks but it will be at least one month. It is very difficult to know at this moment how many weeks it can be."
Aguero's absence is an obvious blow to City with the 25-year-old having been in outstanding form this season, with 19 goals to his name.
Aguero is certain to miss the Christmas programme, which includes a home clash with Liverpool in the Barclays Premier League, and could also be out for both Capital One Cup semi-final legs in January.