Agar, who is Australia's top-ranked bowler in the format, was the unlucky omission that allowed selectors to unleash Josh Hazlewood, Mitchell Starc and Pat Cummins in the same T20I attack for the first time. He has been a mainstay in Australia's T20I side during recent years, featuring in 24 of the 30 matches since his recall in 2018, with the only games he has missed being through injury.
Coach Justin Langer usually picks five specialist bowlers in his XI but there was only room for the three quicks and Adam Zampa in Abu Dhabi, where Stoinis stroked the winning runs in the final over of a stop-start chase of 119.
Glenn Maxwell delivered four overs - the first time he had completed his allocation in a T20I since 2018 - including the second of the match that featured the dismissal of Temba Bavuma, as Australia did well to restrict South Africa to 118 for 9.
"There's an argument for everyone to be in the team," Stoinis said. "There's no easy answers and we'll probably just see what happens as the tournament goes on. I think everyone's going to get used in this tournament.
"With these conditions, we've got to adapt as we go. He [Agar] has got such a good record, has been so good for us. So I wouldn't be counting anything out."
Kane Richardson, Mitchell Swepson and Josh Inglis were the other members of Australia's 15-man squad that didn't make the cut in their tournament opener.
Aaron Finch said after winning the toss they were on the wrong side of "brutally tough" calls. Finch, who didn't ask Stoinis or Mitchell Marsh to deliver a single over, expanded on the logic of off-spinning allrounder Maxwell's key role with the ball after the five-wicket victory.
"We felt there were some really good match-ups in the powerplay," Finch said. "He did a really good job."
Before the tournament, Maxwell had talked about how he had been working on his bowling to right-handers to make himself a more valuable option for Finch. "I suppose that's the biggest thing I've been working really hard on, bowling from over the wicket and being able to attack right-handed batters and it feels like it's going really well," Maxwell said.
Stoinis agreed and said Maxwell's innovative approach to batting was part of what made him a "brilliant T20 bowler".
"One of the benefits of batting like he does, he understands the game and he understands what batters are trying to do," Stoinis said. "He's very smart. He's obviously played so much.
"I definitely think he's someone we should be using in this World Cup. He did match up well to a few of their left-handers and the wickets suited it, but there's no reason why he won't be bowling."