The Pacific islanders eventually ran out 43-10 winners over the Chileans but Mapusua said they could not afford a similarly sluggish start on Sunday against the Pumas, who have twice reached the World Cup semi-finals.
"The boys realise that we're in the tournament now, we've arrived at the party and there are no second chances," Mapusua said.
"So this week in the whole squad there's been a real lift and an energy and we know that we've got a huge game against the Pumas on Friday. We won't be short of motivation."
Argentina lost their opener to England and will be desperate to get their campaign back on track at Stade Geoffroy-Guichard on Sunday.
"We are under no illusion about the threat that Argentina poses and we believe that if we stick to how we want to play and play the Manu Samoa way then I'm confident that we'll be able to stop Argentina," Mapusua added.
Mapusua made three changes to his starting line-up from the team that beat Chile with prop Paul Alo-Emile and Brian Alainu'uese coming into the pack and the powerful Ben Lam named on the right wing after recovering from injury.
"He's a very big man who can move quickly and is a genuine threat with the ball in hand," Mapusua said of Lam.
"He brings a lot into the wide channels, just his pure power and the way he plays the game so we're looking forward to finally getting him on the field this weekend."
Former All Blacks prop Charlie Faumuina looks set to play his first World Cup match for Samoa in Friday's match after being named on the bench.
The 2015 World Cup winner was one of two former New Zealand forwards named in the matchday squad along with Steven Luatua but former All Blacks first five Lima Sopoaga misses out with a calf strain.
Christian Leali'ifano retains the number 10 jersey for the match against an Argentina side led by his former Wallabies coach Michael Cheika.
"I'm glad he's not playing," Leali'ifano joked.
Team: 15-Duncan Paia'aua, 14-Ben Lam, 13-Ulupano Junior Seuteni, 12-Tumua Manu, 11-Nigel Ah-Wong, 10-Christian Leali'ifano, 9-Jonathan Taumateine, 8-Steven Luatua, 7-Fritz Lee, 6-Theo McFarland, 5-Brian Alainu'uese, 4-Chris Vui (co-captain), 3-Paul Alo-Emile, 2-Seilala Lam, 1-James Lay
Replacements: 16-Sama Malolo, 17-Charlie Faumuina, 18-Michael Alaalatoa (co-captain), 19-Taleni Seu, 20-Sa Jordan Taufua, 21-Melani Matavao, 22-Alai D'Angelo Leuila, 23-Danny Toala.