The Argentina striker has been missing as title-chasing City have dropped potentially vital points in their last two Barclays Premier League matches, including Saturday's 3-3 home draw against Sunderland. Mancini has been reluctant to discuss the problem, only referring to it as "stupid".
But the manager did appear to confirm reports that the player's skin reacted badly to a spray used to treat a knock and - when asked if the spray was involved - Mancini said: "Yes."
Mancini had spent the preceding moments trying to downplay the situation and again insisted no blame was attached to the 24-goal striker over the matter.
He said: "It was not his fault, like someone wrote in the newspaper this morning."
Mancini claimed some theories published as to how the initial injury might have been caused are "not true".
What he could not deny, however, was that the 23-year-old was badly missed as City missed the chance to move back to the top of the table.
Rivals Manchester United could now pull five points clear with victory over Blackburn on Monday.
Mancini said: "We missed Aguero and for this reason I am disappointed. I hope he can recover for the next game."