The striker sustained ligament damage in his knee during Sunday's 3-2 win over Southampton which required him to be substituted in the first half.
Fears of serious damage were yesterday allayed by City who confirmed he would return to action after the international break after the first week in September.
Despite that, the Argentina Football Association have selected Aguero for their 2014 qualifiers against Paraguay in Cordoba on September 7 and Peru in Lima five days later which Mancini cannot believe.
"I don't understand this, I don't understand," he said.
"The national manager (Alejandro Sabella) should understand this is a serious case. A bad injury.
"These games are really strange because all the players are not ready to go. I don't understand this. Sometimes the players are not ready for this game.
"Aguero is not so bad like we thought. After the international break he will be ready. Two or three weeks.
"It is a bad injury but not for six months. After two weeks he can start to train maybe."