The 24-year-old Montenegro international has made just five appearances so far this season.
"For the player it is always very bad news when he cannot play, but I think he is a very important player," said Pellegrini ahead of his side's Capital One Cup semi-final first leg against West Ham on Wednesday.
"He was the best choice to replace Kun Aguero when Kun got injured, but things happen and we must try to be optimistic for the future and suppose Stevan will play in the second part of the season."
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