The South American giants go head to head in Belem in the second leg of the Superclasico de las Americas, organised by the Conmebol, after a goalless first meeting in Cordoba two weeks ago.
Boca coach Julio Falcioni has expressed his concern over excessive workload for his players after three league games in 10 days, but Viatri insists he is fit and cannot wait to represent his country.
"The Argentinian Football Association doctors tested me and I am fine,'' he said.
"I know that I will be very tired if I finally play this match, but representing your country is the best thing you can ever experience.''
Argentina coach Alejandro Sabella was unable to call up overseas-based stars such as Carlos Tevez and Lionel Messi for the fixture, given it is not a date FIFA have set for international duty, but he was able to count on players from South American leagues.
One of the biggest names in the Brazilian league, Flamengo's Ronaldinho, will turn out for the home team.
The 31-year-old was surprised to see 25,000 supporters in attendance for yesterday's training session in Belem.
"I am very happy to be playing for the national team here and I hope I can repay this support in the best way possible,'' he said.
Brazil boss Mano Menezes has yet to confirm his starting line-up, leaving Corinthians midfielder Diego Souza anxiously waiting for his opportunity to make his mark ahead of the 2014 World Cup on home soil.
"Mano is the one who defines the line-up, but expectation is huge,'' he said.
"The national team is going through a transitional period and opportunities will come up. I need to be ready.''
Some of the players who had been included in the squad for this match will miss out due to injury, while Gremio defender Mario Fernandez has rejected the call-up.
Corinthians midfielder Paulinho will not play for Brazil due to a left shin injury.
National team doctor Rodrigo Lasmar said: "The tests showed it is not a serious injury, but demands progressive treatment.''
On the other side, Andres D'Alessandro, who plays in Brazil for Internacional, will not play for Argentina due to a left knee problem, while third goalkeeper Esteban Andrada, who plays at Lanus, will not travel to Belem.
Walter Montillo, another of the Argentinians playing in Brazil, was unable to hide his delight after being given his first call-up to the Albicelestes' squad.
"This is a dream come true," he said. "I still feel like I'm flying. I hope I can help either on or off the field, but I must take advantage of this chance.''
Montillo is confident in his team's chances tomorrow, saying: "Brazil change a lot. In terms of the players, the coach, they change. If you attack them properly, you can beat them.''