Scaloni made similar comments ahead of Argentina’s last game, a 1-0 win against Uruguay on Friday, although Messi only appeared for the final 14 minutes of the match.
Argentina are second in South America’s 10-team qualifying group, six points behind Brazil, who are the only team from the region to have secured their place at Qatar 2022 so far.
Scaloni repeated that Messi, 34, had recovered fully from the knee and hamstring worries that kept him out of PSG's last two games and were ready for one of the biggest matches in the South American calendar.
“He was physically fit the other day and in the end, we decided that the best thing was for him to play a few minutes so that he could get a feel for it and he is confirmed to play tomorrow,” Scaloni said of Tuesday’s match in San Juan.
“It’s going to be a very difficult game,” said Scaloni. “They are the team that leads the qualifiers and they are already through to the finals."
The top four teams in South American qualifying progress automatically to next year's World Cup finals and the fifth-placed side goes into an inter-regional playoff.
Argentina are unbeaten in 26 games, a run that includes a 1-0 win over Brazil in the Copa America final in Rio de Janeiro in July.