Roberto Mancini has revealed Sergio Aguero could be out for up to two weeks after sustaining a "stupid injury".
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Aguero, who missed Manchester City's 1-1 draw with Stoke last weekend, is now certain to miss Saturday's home game against Sunderland.
City have been coy about the precise nature of the problem but, in Friday's press conference, Mancini said it could be worse than first feared.
"It is a stupid injury," Mancini said. "It was not his fault but he can't play with this injury - for one day, ten days or two weeks. I don't know."
Mancini was pressed on the specifics of the injury several times but each time refused to elaborate.
"I prefer not to say," he said, "but he couldn't play against Stoke and can't play tomorrow. We hope we can recover him for Arsenal but I don't know if it is possible. I am really disappointed and I hope it won't be weeks."
Captain Vincent Kompany is set to return for Saturday's game, but Joleon Lescott remains sidelined.
Mancini was also asked about Mario Balotelli, with the striker having made a surprise appearance at an Inter Milan press conference during the week. "I think he was there as a journalist," Mancini said with a smile. "It is better you ask him why but he had two days off."
Meanwhile, Mancini decided not to dwell on Patrick Vieira's comments during the week that big clubs - notably Manchester United - can get better treatment from referees.
"Sometimes it can happen (that big teams get decisions)," Mancini said. "It is normal everywhere, but I don't want to talk about what other people think. Patrick is a clever man. He knows what he wants to say."