Winning the toss and electing to bowl first, captain Mitch Marsh indicated left-arm quick Starc would be the man to drop out given expectations of a spin-friendly pitch in St Vincent.
Agar comes in for his second match of the tournament after he featured in the defeat of Scotland in the group stage.
Australia also rested Starc from the group match against Namibia after he pulled up sore between games, but Marsh said Sunday's selection call was purely conditions-based.
A victory for Marsh's side would end Afghanistan's push for a spot in the final four and confirm Australia and India as qualifying from the first Super Eight group.
Afghanistan have made two changes to the side that lost to India in their previous game, bringing in Nangeyalia Kharote for his first match of the tournament proper and recalling fellow allrounder Karim Janat.
Specialist batters Hazratullah Zazai and Najibullah Zadran drop out.
The match marks Australia's first clash against Afghanistan since Glenn Maxwell's legendary double-century secured a famous comeback win and a semi-final berth at the 2023 one-day international World Cup.
TEAMS:
Australia: David Warner, Travis Head, Mitchell Marsh (c), Glenn Maxwell, Marcus Stoinis, Tim David, Matthew Wade, Pat Cummins, Ashton Agar, Adam Zampa, Josh Hazlewood
Afghanistan: Rahmanullah Gurbaz, Ibrahim Zadran, Gulbadin Naib, Azmatullah Omarzai, Mohammad Nabi, Karim Janat, Nangeyalia Kharote, Rashid Khan (c), Noor Ahmad, Naveen-ul-Haq, Fazalhaq Farooqi
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