Sanchez celebrated his 100th cap with a flawless performance in the Pumas's 59-5 demolition of Chile in their previous outing, but coach Michael Cheika kept faith in Carreras for his team's decider against Japan.
The winners of the game at La Beaujoire stadium will go through to the quarter-finals alongside group winners England and the losers will pack and go home.
Argentina will go through on points difference should the game end up in a draw, unless Japan take a bonus point for scoring at least four tries and the Pumas do not.
Samoa and Chile are already eliminated.
Since their opening defeat, the Pumas have been playing with the fear of elimination, meaning they are ready to deal with the pressure that comes with the Japan game.
"This is why you play rugby. If you want it easy and for the script to be written, don't worry about coming," Cheika said.
"In international rugby, you don't always have games where everything is at stake. We have been doing it since the game against England.
"You have to have a certain level of pressure. They are feeling that pressure for the first time and they are growing. It will be the third elimination match and we hope to have three more from now on."
Tomas Lavanini is back in the second row after nursing a leg injury while prop Francisco Gomez Kodela and centre Lucio Cinti will also start.
"We think it has good balance, not only in the starting XV but also in the eight that finish the game," Cheika explained.
"We are happy with the work we’ve done, the competition within the squad. Choosing the team was difficult."
While Argentina were twice in the semi-finals, they failed to go through the group stage in 2019, a feat Japan achieved.
"We know that they were in the quarter-finals in the last World Cup and not us, so we want to change this situation," Cheika said.
Argentina team:
1-Thomas Gallo, 2-Julian Montoya (captain), 3-Francisco Gomez Kodela, 4-Guido Petti Pagadizabal, 5-Tomas Lavanini, 6-Pablo Matera, 7-Marcos Kremer, 8-Juan Martin Gonzalez, 9-Gonzalo Bertranou, 10-Santiago Carreras, 11 Mateo Carreras, 12-Santiago Chocobares, 13-Lucio Cinti, 14-Emiliano Boffelli, 15-Juan Cruz Mallia
Replacements: 16-Agustin Creevy, 17-Joel Sclavi, 18-Eduardo Bello, 19-Matias Alemanno, 20-Pedro Rubiolo, 21-Lautaro Bazan Velez, 22-Nicolas Sanchez, 23-Matias Moroni
Japan coach Joseph makes one change
Japan have made a single change for the match as Siosaia Fifita comes onto the left wing for the crucial clash.
"The players are in really good condition, both mentally and physically," coach Jamie Joseph said.
"We've had an up and down season, but I feel that we're starting to play some of our best rugby at the right time."
Joseph largely kept faith with the side that beat Samoa 28-22 last time out, with Fifita replacing Jone Naikabula.
Kotaro Matsushima is on the other wing with Lomano Lemeki keeping his place at full-back after an impressive display against the Samoans.
Ryoto Nakamura and Dylan Riley are the centre-pairing, with fly-half Rikiya Matsuda, who has missed a single kick at goal in three games at the tournament, alongside Naoto Saito in the half-back pairing.
Captain Kazuki Himeno is at number eight and part of a loose trio that also includes Michael Leitch and Pieter Labuschagne, while the second row is made up of Jack Cornelsen and Amato Fakatava.
Hooker Shota Horie packs down with props Keita Inagaki and Jiwon Gu.
Japan have lost five of their previous six tests against Argentina, including the last three, but have won nine of their last 11 pool matches at the World Cup.
"We just have to play 80 minutes of rugby and ensure we play as best we can and play our style of rugby," New Zealander Joseph added.
"We've been preparing for this game for the last three years, so we're really looking forward to it."
Japan team:
1-Keita Inagaki, 2-Shota Horie, 3-Jiwon Gu, 4-Jack Cornelsen, 5-Amato Fakatava, 6-Michael Leitch, 7-Pieter Labuschagne, 8-Kazuki Himeno (capt), 9-Naoto Saito, 10-Rikiya Matsuda, 11-Siosaia Fifita, 12-Ryoto Nakamura, 13-Dylan Riley, 14-Kotaro Matsushima, 15-Lomano Lemeki
Replacements: 16-Atsushi Sakate, 17-Craig Millar, 18-Asaeli Ai Valu, 19-Warner Dearns, 20-Amanaki Saumaki, 21-Yutaka Nagare, 22-Ryohei Yamanaka, 23-Jone Naikabula.
