Mostert, who is headed to play for Gloucester this season after captaining the Lions to this month's Super Rugby final, comes in for Pieter-Steph du Toit in the starting lineup.
“Franco needed rest after the Super Rugby season and the June internationals and we therefore didn’t consider him for Durban. He’s been our first-choice No. 5 lock and that’s why he's starting on Saturday,” said Erasmus, who kept his backline unchanged.
“I want to us to build more continuity and those combinations now have another opportunity to play together and gain more experience,” he said in a statement on Thursday announcing the team.
Erasmus suggested after Saturday’s 34-21 win that he would make four to five changes but has had a change of heart.
There are changes among the forward replacements as lock RG Snyman returns to the matchday squad in place of Marvin Orie, who did not travel, while prop Wilco Louw rotates places with Thomas du Toit.
Pieter-Steph du Toit drops to the bench and takes the place of Marco van Staden, who won his first cap last Saturday.
Assessing Argentina, Erasmus added: “We’ve always rated them as a very physical team and we felt the game (in Durban) was physical. They’re going to step it up and I think they will come hard at our mauling, scrums and the breakdowns.”
Team: 15-Willie le Roux, 14-Makazole Mapimpi, 13-Lukhanyo Am, 12-Andre Esterhuizen, 11-Aphiwe Dyantyi, 10-Handre Pollard, 9-Faf de Klerk, 8-Warren Whiteley, 7-Siya Kolisi (captain), 6-Francois Louw, 5-Franco Mostert, 4-Eben Etzebeth, 3-Frans Malherbe, 2-Malcolm Marx, 1-Tendai Mtawarira Replacements: 16-Bongi Mbonambi, 17-Steven Kitshoff, 18-Wilco Louw, 19-RG Snyman, 20-Pieter-Steph du Toit, 21-Embrose Papier, 22-Lionel Mapoe, 23-Damian Willemse
(Reporting by Mark Gleeson in Cape Town, editing by Toby Davis)
