The Argentine midfielder had been pushing for a return to boyhood club River Plate on loan, but Real Madrid have made clear they would rather he continues developing in Europe, and at least three Italian clubs are ready to negotiate a temporary deal before the new season gets under way.
Serie A has emerged as the most likely destination now that the River Plate option has been shelved. Fiorentina have already opened talks with Real Madrid, while Napoli and AC Milan have also been tracking the 19-year-old for some time.
The identity of the club matters less than finding the right project: Mastantuono is looking for regular game time and a role playing off the main striker as an attacking midfielder, the position in which he feels most comfortable but rarely occupied last season, when he was used mainly on the right wing.
Mastantuono joined Madrid from River Plate for around €45m last year and signed a six-year contract that runs to 2031, complete with a €1 billion release clause reflecting the scale of the club's investment in him. He made 35 appearances in his debut season but managed just three goals and one assist, never fully establishing himself under Xabi Alonso, and missed out on Argentina's World Cup squad as a result.
Following talks between the player and Jose Mourinho, who now oversees the club's sporting project, the conclusion reached was that a loan offers the clearest route to the regular football that could not be guaranteed at the Bernabeu this season — with Real Madrid unwilling to include any option to buy given the size of their outlay.
Dean Huijsen's absence from the squad facing Fiorentina is a rather different story. The centre-back has become Real Madrid's second injury concern of pre-season after picking up a muscle problem that is expected to keep him sidelined for around two weeks, following a similar setback for Raul Asencio in the same period.
It is a frustrating landing spot for Huijsen, who arrived at the club last year in a deal worth close to €60m and had been expected to be a key figure in Real's backline from the opening weeks of the campaign.