The Argentine striker had to be withdrawn at half-time during his side's 2-1 Champions League defeat at Barcelona and is expected to be out "for more than one week"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 half-time at Camp Nou on Wednesday, as City fell to a 2-1 defeat to Barcelona in the Champions League, after damaging his hamstring.
However, Pellegrini was upbeat in his pre-match press conference, confirming that Aguero "will be out for more than one week" but insisting that the injury is not serious.
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Despite setbacks against both Barca and Wigan - to whom City lost in the FA Cup sixth round - the Chilean 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 continued.
"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 'til 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."