[5] Mastantuono continued his progression through the River Plate academy, and in August 2023, he signed his first contract with the club – a two-year deal with a €30 million release clause.

[7] The following year, Mastantuono was included in River Plate's squad for their pre-season preparations, having been called up alongside a number of other youth team players by coach Martín Demichelis.

[11] Having established himself as a fixture in the first-team setup, calls from fans for Mastantuono to play more were ignored by coach Martín Demichelis, with former River Plate defender Reinaldo Merlo stating that, while he had shown his ability, he was not ready to feature regularly.

[4] River Plate reserve team assistant manager, Pablo Fernández, has also lauded his ball-striking, as well as his ability to score from free-kicks and beat players one-on-one.

[4] Mastantuono himself has stated that he looks up to River Plate forwards Ignacio Fernández and Matías Suárez, as well as fellow Argentine Julián Álvarez, and tries to model his style of play on them.