Messi, who has said he will retire before the 2026 World Cup, was included in the squad despite carrying a muscle problem which has kept him out of action for club side Inter Miami.
The forward came on in the 53rd minute of Argentina's 1-0 win over Paraguay on Thursday and went close to scoring, rattling the woodwork with a free-kick in added time.
"Leo is fine," Scaloni told reporters ahead of the game in Lima. "He has been adding minutes of training. We will make the decision tomorrow. It's a matter of minutes, of how much he can play. If he's well, you know what I think, he will play.
"We always try to play those that are at 100% or close to 100%. Today it is very difficult for everyone to be at their best in these games so there may be a variation with respect to the game against Paraguay.
"I think the team plays in a determined way, whoever is on the pitch, that's the most important thing. We saw the other day we can play well too (without Messi)."
Asked if Argentina should get used to playing without their talisman, Scaloni shot back: "Let's keep in mind that he is still here. What a way to think about 'when he is gone'.
"The truth is that he is still active, let's leave him alone, are we already retiring him?"
Argentina top the CONMEBOL standings with nine points from three matches, two points ahead of Brazil.
