Pellegrini, speaking ahead of his side's crucial Champions League clash with Roma on Wednesday night, revealed that Aguero will be sidelined until the start of 2015.
"He will be out for four to six weeks," said Pellegrini, "His recovery will depend. But our team does not depend on one player."
Aguero went to ground clutching his knee against Everton on Saturday, and was seen leaving the field in tears after yet another injury setback.
The former Atletico Madrid striker was in a remarkable run of form, hitting eight goals and three assists in his last eight games for City.
Yet now, the 26-year-old will be on the treatment table once again, which is becoming a recurring theme for Aguero and City.
Pellegrini now has to rely on his two remaining centre forwards, Edin Dzeko and Stevan Jovetic, whilst his decision to allow Alvaro Negredo to join Valencia on loan in September looks to have come back to haunt him with the latest news surrounding Aguero's injury.