The Argentine ace injured his calf in Saturday's 6-3 win over Arsenal and may now miss up to eight weeks in what could be a major blow for City.
"I am not a doctor. It is very difficult," said Pellegrini after City's 3-1 Capital One Cup quarter-final win over Leicester City.
"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."
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