The seven-time Ballon d’Or winner had battled an Achilles injury before the tournament and sparked further fitness concerns after missing the first part of open training on Friday and then working on his own on Saturday.
However, Messi insists that was merely a precautionary measure and that he is ready to start in the Group C contest in Lusail.
"I feel very good physically, I think I'm in a great moment, both personally and physically and I don't have any problems," Messi said at a press conference on Monday.
"I heard that they said I trained differently. It was because I had a knock but there is nothing strange (happening). It was just a precaution."
Argentina have already sent Joaquin Correa and Nico Gonzalez home from the World Cup due to injury but still have an embarrassment of riches to pick from in attack.
Julian Alvarez and Lautaro Martinez are competing to spearhead a forward line, and it is the latter who gets the nod.
Lionel Messi trains alone before World Cup
Aston Villa’s Emiliano Martinez starts in goal and Tottenham defender Cristian Romero overcomes fitness concerns to start.
Argentina XI: E Martinez; Molina, Romero, Otamendi, Tagliafico; De Paul, Paredes, Papu Gomez; Messi, La. Martinez, Di Maria.