Ross Cronje replaces Francois Hougaard in the number nine shirt in an otherwise unchanged backline after recovering from an ankle injury that prevented him from playing in the 41-23 victory over Argentina two weeks ago.
Coetzee's decision to rest Mostert means Pieter-Steph du Toit will start in the second row alongside captain Eben Etzebeth, who will play his 60th test on Saturday.
"Franco has played an enormous amount of rugby - at Super Rugby level, playing club rugby in Japan and starting all of our test matches so far this year," said Coetzee.
"We have to be sensible in managing the workload of certain players, such as Franco. We are fortunate to have a number of very good locks. Pieter-Steph now gets an opportunity to start again, and we know what he is capable of."
Lood de Jager will provide cover for the second row from a bench which also features flyhalf Handre Pollard, who has not played a test match since the 2015 World Cup because of a string of injuries.
"I am really satisfied with the way he has trained till now and it's great for the team to have a player of his calibre on the bench," Coetzee added.
A resurgent Springboks side will be looking for a sixth straight test victory in Perth after sweeping France 3-0 in June and beating Argentina home and away in their first two tests in the Rugby Championship.
Team: 15-Andries Coetzee, 14-Raymond Rhule, 13-Jesse Kriel, 12-Jan Serfontein, 11-Courtnall Skosan, 10-Elton Jantjies, 9-Ross Cronje, 8-Uzair Cassiem, 7-Jaco Kriel, 6-Siya Kolisi, 5-Pieter-Steph du Toit, 4-Eben Etzebeth (captain), 3-Coenie Oosthuizen, 2-Malcolm Marx, 1-Tendai Mtawarira.
Replacements: 16-Bongi Mbonambi, 17-Steven Kitshoff, 18-Trevor Nyakane, 19-Lood de Jager, 20-Jean-Luc du Preez, 21-Francois Hougaard, 22-Handre Pollard, 23-Damian de Allende.
(Reporting by Nick Mulvenney, editing by Greg Stutchbury)
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