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Mastantuono received his first senior call-up for Argentina's FIFA World Cup qualifiers against Colombia and Chile in June, making his debut in a 1-0 win against the latter.

His first appearance for the Albiceleste saw him become his nation's youngest official debutant at 17 years and 296 days old, surpassing Adolfo Heisinger's previous record.

The talented teenager has also been included in Lionel Scaloni's latest Argentina squad for their international friendlies versus Venezuela and Puerto Rico.

Mastantuono is hoping to impress the Argentina boss to put his case forward for a spot in its FIFA World Cup squad, with the tournament taking place next summer.

"It would be a dream," Mastantuono stated. "I'm going to work hard to get to the 2026 World Cup. I'm taking it step by step.

"I can play there at 18. That would be an incredible dream. I feel very comfortable with the group. It's my goal to make it to the [FIFA] World Cup."

Messi was absent for Argentina's 1-0 defeat to Ecuador in its final FIFA World Cup qualifier, having scored twice in a 3-0 win over Venezuela five days earlier.

Mastantuono replaced Leandro Paredes in the 62nd minute of that loss at Estadio Monumental Banco Pichincha, donning Messi's iconic number 10 shirt.

"Using the number 10 is incredible," he said. "As I always say, wearing Leo's 10, a number that Maradona also wore, is the best. I didn't expect it; it was something unique.

"Every time I see a photo of me wearing that shirt, I get very emotional.

"I went into the locker room, went to my No. 21 locker, and saw my name next to Julian [Alvarez], and I thought they were messing with me. It was a unique moment.

"I have the date [of the 2022 FIFA World Cup final] tattooed on my body.

"It's a way to remember that Argentine national team, but especially for the thorny path Leo had to walk to get the title. It was the best thing that could have happened to us.

"Now having Messi as a team-mate is the best. I've always admired him. I've always dreamed of playing football; I never thought I'd be able to experience all this."

Mastantuono joined Madrid in pre-season from River Plate for a reported €45 million ($79.4 million) fee, and he has started on 10 of his 12 appearances for the club across all competitions.

Those outings have seen him score just once from the right wing, a role he believes is different in style and position from his work with the national team.

"I like to play wide right, or in the middle, but I'll play wherever necessary, even at centre-back," Mastantuono joked.

"The difference between Real Madrid and Argentina, in terms of play, is clear.

"At my club, we strive to be more direct, while with the national team, we play more with the ball, with possession."