Aguero has yet to play for his new club since completing a £38 million ($62 million) move from Atletico Madrid last week.
But Mancini said the Argentina international forward, who suffered an injury scare in his first training session with his new club, will be on the substitutes' bench.
"Sergio is ok. He had a problem, but yesterday (Thursday) he did 20 minutes training," Mancini told reporters at City's training ground on Friday.
"But today he will train and he will be available for the bench. I don't think he's 100 percent ready but he's fit to be on the bench."
Aguero's compatriot Carlos Tevez will not feature in the Community Shield, the traditional curtain-raiser to the English season between last term's Premier League champions (United) and the FA Cup winners (City).
Former United striker Tevez is unhappy at City and wants to leave, with Inter Milan among the clubs linked with the forward.
Asked whether Tevez will play for City again, Mancini said: "He is a City player. At the moment we don't have any news about other things and Carlos will play for Manchester City.
"Carlos is a professional player and if he doesn't find another team in a few weeks then he will play for us," the Italian added.
"If he stays here I will be happy."
Sunday's match is a muscle-flexing exercise rather than a title pointer but Mancini believes big-spending City can be an even bigger force this season than last when they ended a 35-year wait for a major trophy by winning the FA Cup.
City will take part in the Champions League for the first time this term but Mancini has his eye on the domestic title, although he admits they still have some catching up to do.
"Manchester United are at the top because they are a strong team," he added. "They have bought a few players. I think we are very close but United are five yards ahead of us."
Mancini added he was still in the market for "two or three" other players before the transfer window closes.
Samir Nasri is a target but Mancini, when asked about the Arsenal midfielder, replied: "It's better we don't talk about this."
Source: AFP
Source: AFP