The Argentine striker had to be withdrawn at halftime during his side's 2-1 Champions League defeat at Barcelona and is expected to be out "for more than one week."
The Argentine was replaced at halftime at Camp Nou on Wednesday, as City fell to a 2-1 defeat to Barcelona in the Champions League, after injuring his hamstring.
However, Pellegrini was upbeat in his press conference Friday, confirming that Aguero "will be out for more than one week" but saying that the injury is not serious.
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Despite setbacks against Barca and Wigan — to whom City lost in the FA Cup sixth round — the manager is determined to push as hard as possible to win the Premier League, though he expects that his side would need to "win all the games" left in order to do so.
"We didn't expect to go out against Wigan — now we try to fight for the Premier League title," Pellegrini said. "I don't think it's just only Chelsea [in the race], there's Arsenal and Liverpool, [but] our team will fight till the end. We need to win all the games till the end of the season.
With a word of caution, he added: "The game against Hull [City on Saturday] is special as we have less than 72 hours to recover."