Etzebeth suffered a head knock in training ahead of last week's Nations Championship victory over England, and given his previous concussion issues, the Boks are taking a cautious approach with his recovery.
"Eben needs some proper time off because of concussion, and he has a history of concussion, so I don't think Eben will play in these incoming matches," Erasmus confirmed on Monday.
The double World Cup-winning lock has endured a disrupted year, missing extended periods of the Vodacom URC season with the Sharks due to a combination of injuries and other setbacks.
After serving a 12-match suspension following the Boks' end-of-year tour clash against Wales, Etzebeth suffered a hip injury in his first match back, keeping him sidelined for another two months.
Mostert, meanwhile, has also been sidelined following an ankle injury suffered against the Barbarians last month, with Erasmus confirming that the veteran lock is on a similar timeline as Etzebeth.
The duo are in line to return in time for the Boks' one-off clash against Argentina on August 8, just before the start of the Greatest Rivalry series against the All Blacks.
"We are targeting Argentina for Eben's return. The same goes for 'Sous' [Franco Mostert]," Erasmus added.
In more positive news, Kolisi is expected back next week following a hamstring injury, while Lood de Jager, Ethan Hooker, Ox Nche, Sacha Feinberg-Mngomezulu and Morne van den Berg are all making good progress in their recoveries from hip, shoulder, knee, ankle and bicep injuries respectively.
"Lood might be ready next week for the Wales game in Durban, he is closer than the others. Ethan could be available," said Erasmus.
"Ox, Sacha, Krappie, all of them will be ready about three weeks before New Zealand. Siya next week."
Scotland lock Scott Cummings believes his side can challenge the Springboks when the teams meet at Loftus Versfeld on Saturday, despite South Africa's formidable record on home soil.
Rassie Erasmus has made 10 changes to his starting lineup for the Springboks' Nations Championship clash against Scotland, but it's still a formidable team that could compete with just about anyone on the international scene.
After a long wait for his return to the Springbok starting lineup, Embrose Papier admits he is feeling a mix of nerves and excitement ahead of Saturday's clash.
Springbok coach Rassie Erasmus has made ten changes to his starting team for Saturday's Nations Championship Test against Scotland in Pretoria.
