Goalkeeper Hart believes that the duo will have a big impact on City's campaign on all fronts.
"Thirty-odd million is a big price tag but it is already looking like money well spent," Hart told Sky Sports News.
"We haven't seen too much of him because the manager has kept him quite well protected as he has had a big summer.
"But with the likes of Sergio coming in and David Silva, who has to get a mention, we will have a decent season."
Aguero, 23, started among the substitutes after manager Roberto Mancini revealed that he wasn't match fit.