Aguero missed the draws with Stoke at Britannia Stadium and Sunderland at Etihad Stadium - City's first dropped points at home this season - after suffering an allergic reaction to a spray he used to treat an ankle problem.
The four dropped points have seen City slip behind Manchester United in the title race, but Mancini hopes the return of his top scorer will bring with it a change in fortunes.
"In just three or four days he is fit," Mancini said. "He started to train for the first time [on Wednesday] and today [Thursday] and [will train on Friday]. After that he will be fit."
Meanwhile, Mancini has suggested Mario Balotelli's behaviour on and off the pitch is blown out of all proportion.
Balotelli has proved a constant source of headlines throughout his two seasons at City and has been on front as well as back pages of newspapers this week.
Mancini said: "There are some people who think bad things against other people, like Mario. Every time Mario does something, (it) is like a war.
"There are other people on and off the pitch that do worse than Mario and no-one says anything because maybe they play for an important team or are not like Mario."
Asked if he tires of answering questions about his fellow Italian, Mancini said: "I would like Mario to play, score and finish the other situation.
"It is not maybe your [the media's] fault - it is his fault because if he wants he can play and stay at home. Like this, no people can ask about the other situation. I think many people have different behaviour with Mario."
There have been suggestions that Balotelli will be offloaded in the summer, with AC Milan and former club Inter Milan said to be interested.
Inter director Marco Branca was asked about Balotelli at a press conference on Thursday, but would only say: "There will be time to speak about this in one or two months."