The Argentina international, who recently missed eight matches with a calf problem, was forced off in the first half of City's 5-1 victory at Tottenham Hotspur in the English Premier League on Wednesday. "The doctor is seeing him - maybe one month, but we are not sure absolutely," Pellegrini told reporters.
The injury to Aguero, who has scored 26 goals this season, arrives at an unwelcome juncture in City's campaign, with the 2012 English champions having just taken over from Arsenal at the top of the league table.
He stands to miss the first leg of City's UEFA Champions League last 16 tie with Barcelona, as well as key league and FA Cup games against Chelsea. However, with City having already amassed 115 goals in all competitions this season, Pellegrini believes that his team can cope with Aguero's absence.
"It is very difficult. Sergio is a top player for us, but I think we can replace him," said the Chilean. "As with the other injury, we hope we can continue to do it. But, of course, he is a very important player for us. Sergio is just coming back from an injury -- one month that he couldn't play. All the other strikers - Alvaro Negredo, Edin Dzeko and now Stevan Jovetic - replace him."