Ramires, 27, was withdrawn from Dunga's squad on Thursday due to a muscle problem that will see him miss the Argentina clash on October 11, and the Japan friendly three days later.
Defensive midfielder Souza, who is on loan at Sao Paulo from Gremio, has been called up to replace Ramires.
Dunga has been hit with injury worries for what will be his third and fourth matches in charge since beginning his second stint at the helm in July.
Goalkeeper Jefferson hurt his finger during Botafogo's 3-2 loss to Santos in the Copa do Brasil on Wednesday.
Brazil are confident Jefferson will be fit for the friendlies, but have called up Gremio goalkeeper Marcelo Grohe as a precaution.
Grohe has kept eight consecutive clean sheets for Luiz Felipe Scolari's in-form Gremio side.
