The match, which is scheduled to kick off at 15h00 and has been moved to Newlands, will be broadcast live on SuperSport.
Mngomezulu will take over at flyhalf from Jordan Hendrikse from the side that outplayed Argentina 33-26 earlier in the series, while Alexander, who missed that match due to a leg strain, takes over from Bradley Davids at scrumhalf.
Hendrikse, who kicked 18 points against Los Pumitas, has been released back to the Emirates Lions for their clash against the British & Irish Lions on Saturday.
Nhleko also named Dalvon Blood in place of the injured Canan Moodie on the wing, while the coach opted for Jacques Goosen at flank in place of Sasko Ndlovu, who will play off the bench.
Goosen played at hooker in the first three matches, but Nhleko explained that he was picked for a specific role in mind against Argentina.
“We have against played them and have seen their matches against Georgia and Uruguay, so we have a good idea of what they can do and where their strong points and weaknesses are,” said Nhleko.
“Every player is aware of what is expected of him during the match and how they contribute to our collective goal of making the team better. We have picked Jacques to fulfil a very specific role of us.
“The call might seem a bit strange, but the Springboks used Schalk Brits at No 8 at the Rugby World Cup in Japan in 2019, and that served the bigger picture in the Boks’ effort very well.
“We hope to use Sasko later in the game, when it opens up and becomes a bit more free-flowing and that we will benefit from his strong points in that regard.”
The coach confirmed that the 33-26-win last week has been shelved in the memory banks.
“We need to be much better than that day, in fact, I would say we will need to play at least 30% better if we want to achieve the result we are hoping for. The boys have prepared well and are excited at the prospect of another opportunity to improve on our series performance,” he said.
“Argentina have shown their prowess in the tight exchanges and set plays and once they dominate, they become pretty direct, so we really need to make a statement there if we don't want to chase them around all day.”
The decision to move the final round of matches (which will also see Georgia and Uruguay in action at 13h00) to Newlands, due to the state of the field at Markötter Stadium in Stellenbosch, was received with great joy by the Junior Boks, according to Nhleko.
“The opportunity to play at Newlands is a welcome boost and when the players were informed of the venue change, you could feel the energy surge,” he said.
“The last time the Junior Springboks played there, was the Junior World Championship final in 2012, which we won, and we will now most likely be the last South African national team to play there, so it will be huge for the players. We have told them about the history of the field, not only for the home team, but the Springboks as well.”
15. Quan Horn
14. Dalvon Blood
13. Henco van Wyk
12. Tyler Bocks (vice-captain)
11. Ethan James
10. Sacha Mngomezulu
9. Bobby Alexander
8. George Cronje (captain)
7. Siya Ningiza
6. Jacques Goosen
5. Simon Miller
4. Reinhardt Ludwig
3. Corné Weilbach
2. Andre-Hugo Venter
1. Jan-Hendrik Wessels
Replacements: Blaine Golden, Nolan Pienaar, Sibusiso Shongwe (nominated front row), Dylan de Leeuw, Raynard Roets, Sasko Ndhlovu, Izan Esterhuizen, Rynard Mouton, Nico Steyn, Bradley Davids, Kian Meadon, Mnqobi Mkhize, Curwin Gertse.