Aguero was missing from the City team that drew at Stoke on Saturday with a foot problem.
Mancini has refused to say exactly what happened or for how long the Argentina forward could be sidelined, 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 is considering handing striker Carlos Tevez his first league start for almost seven months at Eastlands on Saturday.
However the Italian handler faces a possible dilemma as the Argentinian is still not considered fit enough to start, Edin Dzeko has been inconsistent and Mario Balotelli remains an enigmatic presence in the Blues squad.
Having spent most of the season at the top of the table, City trail leaders Manchester United by three points with eight games remaining, and have had a week to forget.
After City's 1-1 draw last Saturday, United piled on the pressure by defeating Fulham on Monday before Mancini was pictured giving controversial striker Mario Balotelli a dressing down in training on Wednesday.
City haven't dropped points in front of their own fans for 13 months since a 1-1 draw with Fulham, with 20 consecutive victories since then however Mancini's men slid to a 1-0 defeat at Sunderland's Stadium of Light in January.