The rival nations meet for the first time since this year's Copa America final, which Argentina won 1-0, when they face off in San Juan on Wednesday (AEDT).
Both teams have identical records since that final, each winning five of their six qualifiers. A clash between the pair in Sao Paulo in September was suspended due to coronavirus restrictions relating to Argentina's England-based players.
Already-qualified Brazil sit top of the CONMEBOL standings on 34 points, six clear of Scaloni's team, who defeated Uruguay on Friday.
Given the 12-point gap between Argentina and fifth-placed Colombia, who occupy the play-off spot, a win would all but guarantee their place in Qatar next year. Dropped points for two of Chile, versus Ecuador, Colombia, against Paraguay, and Uruguay, in Bolivia, would make the picture clearer still.
Messi, whose selection in Argentina's squad "did not make sense" – according to Paris Saint-Germain sporting director Leonardo – due to the 34-year-old's recent injury issues, came on as a 76th-minute substitute in the win over Uruguay.
Scaloni explained he had used Messi sparingly in order to keep him in top condition for Tuesday's clash with Brazil, and the coach confirmed in his pre-match news conference the former Barcelona superstar would certainly play, albeit he did not reveal if he would start.
"It is confirmed that Messi will play tomorrow," Scaloni said.
On the challenge his team face, Scaloni said: "This Brazilian team is one of the most direct in recent times.
"They have already qualified for the World Cup, and we know how difficult this game is going to be.
"You have to always play the same, it is not worth it for me that a player of mine plays in one way against Venezuela and another against Brazil. Football is always the same regardless of the rival.
"I don't think the rest of the rivals in the qualifiers are less than Brazil. They are all very difficult games.
"There are always things to correct. Our goal is that the players do not relax, that they know that there are things to improve."
While Messi will play, Argentina may be without Paulo Dybala, who was taken off at half-time against Uruguay.
"Paulo came with a blow and at half-time of the game we decided to take him out, it was not worth risking it. Now we are waiting to see what [injury] he has," Scaloni added.