Since then, Mastantuono has only improved. His first senior goal, a sweet volley to seal a win over Excursionistas in the Copa Argentina in February 2024, made him the club's youngest-ever goal-scorer, surpassing a mark then held by Javier Saviola.
Despite that initial promise, a puzzling few months followed for the attacking midfielder. Demichelis was at first reluctant to throw Mastantuono consistently into contests, leading to outrage from local fans as River struggled for form and consistency in Argentina's top-flight. Ultimately, they finished the 2024 season fifth in the standings and Demichelis was shown the door, replaced by club legend Marcelo Gallardo, while Mastantuono was handed another new contract tying him until 2026.
The 2025 season has been much kinder to the teenage, who has worked his way into the starting line up and lived up to the hype. He nearly broke the internet with a wicked Paul Gascoigne-esque free-kick against rivals Boca Juniors in a 2-1 victory back in April, and that was swiftly followed by a first call-up to the Argentina senior team. While with the world champions, he became the men's team's youngest-ever player in a competitive match when he replaced Thiago Almada in a 1-0 victory at Chile.
This summer, the sweepstakes to sign Mastantuono intensified. Newly-crowned European champions Paris Saint-Germain thought they had him, but you can never count out Madrid until a player signs on the dotted line, and it has now been confirmed that he will head to the capital of Spain rather than France in a €40m (£34m/$45m) deal.