Rees-Zammit, 24, is set for his first Test appearance since the 2023 Rugby World Cup, having missed Wales’ summer tour because of his brief switch to American football.
The wing spent 18 months in the United States with the Kansas City Chiefs and Jacksonville Jaguars, but his attempt to break into the NFL ended without a senior appearance.
After signing for Bristol earlier this year, Rees-Zammit made an encouraging start in the Premiership before a toe injury halted his momentum.
Now fully recovered, he has been recalled by Tandy for his first match in charge of Wales — though not straight into the starting XV.
“There's always a temptation to start someone like Zammo, but we've got to do what's right by him and the group,” said Tandy.
“Zammo's probably played two games in total since he's come back and then he's had an injury. It's his first full training week, which has been awesome.
“He's been doing some running and he's been ticking the boxes in lots of the physical aspects, but then you've got (wings) Tommy (Rogers) and Jadzi (Josh Adams) who've trained really well. We've got two boys who are match fit and will give us something.
“Bringing Zammo off the bench does add a massive opportunity to add impact to the game.”
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Tandy, who replaced Warren Gatland and caretaker Matt Sherratt in the summer, has made seven changes to the side that ended Wales’ record 18-match losing streak in Japan in July.
His first selection reflects a balance between rewarding form and easing key players back into the fold.
Rees-Zammit’s inclusion on the bench ensures excitement will be waiting later in the game, with Tandy confident the winger’s pace and finishing instincts can turn tight contests.
“We’ve got to do what’s right by him,” the coach reiterated.
“He’s an exciting player, and when he comes on, he’ll bring something different. The challenge for us is making sure we use that at the right moment.”
The match against Argentina marks a new chapter for Tandy as well as his team.
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Now 44, the former Ospreys flanker will take charge of Wales at the Principality Stadium for the first time as head coach — almost four decades after he served as a ball boy for local club Tonmawr when his father Peter coached them to Welsh Brewers Cup glory at the old National Stadium.
“I was the ball boy (in the Cardiff final), my dad was the coach, and it was Tonmawr's invincible season,” Tandy recalled.
That early exposure to rugby at grassroots level, he said, shaped his philosophy as both player and coach.
“When the boys go out there on Sunday, I want them to play with a little bit of freedom and not worry about every mistake,” he said.
“I'd rather them worry about what they can do.”
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Tandy enjoyed a solid playing career with Neath and the Ospreys before turning to coaching, later working with the Waratahs in Australia and Scotland under Gregor Townsend.
His emphasis on energy, positivity and adaptability has been clear throughout his first weeks in charge of Wales.
“We can’t change our physical attributes but we have got some big aggressive boys as well,” Tandy said of the challenge posed by Argentina’s powerful pack.
“There's some big boys out there.”
Wales’ November schedule is a daunting one, featuring further fixtures against Japan, New Zealand, and world champions South Africa.
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The team have dropped to 12th in the world rankings, and Tandy knows the rebuild will take time — but insists the focus is firmly on growth and performance, not just results.
The Pumas, by contrast, arrive in Cardiff ranked sixth and full of confidence after a strong Rugby Championship campaign in which they beat both Australia and New Zealand, and pushed South Africa all the way in a two-point defeat at Twickenham.
Tandy acknowledged the scale of the challenge but is determined his players embrace it with energy and belief. “We can't change our physical attributes,” he said, “but we have got some big aggressive boys as well.”
