Manchester City's Premier League title bid has suffered a blow after Sergio Aguero was ruled out for an indefinite period by a 'stupid' injury.
Aguero, who has scored 17 league goals this season following his arrival from Atletico Madrid last summer, missed last week's 1-1 draw at Stoke City due to a foot problem.
He has now been ruled out of Saturday's game at home to Sunderland and Mancini is unsure when the Argentine will be fit to return.
The City boss has refused to say exactly what happened, however, it seems the problem was not of an orthodox nature.
"It is a stupid injury," said Mancini.
"It was not his fault but he can't play with this injury; for one day, 10 days or two weeks. I don't know."
Mancini was repeatedly asked what Aguero had done but declined to elaborate each time, adding: "I prefer not to say.
"But he couldn't play against Stoke and can't play tomorrow. I hope he can recover for Arsenal next weekend."
Aguero's continued absence could see compatriot Carlos Tevez make his first start since being brought back into the first-team fold by Mancini.
Tevez has featured as a substitute in the last two games against Chelsea and Stoke, and will be pushing for inclusion when Sunderland visit the Etihad Stadium.