According to CalcioMercato, the Italian club have made contact with Real Madrid over a temporary deal, as Fiorentina's hierarchy focuses this window on adding wingers and attacking midfielders to support their central striker.
Should that interest advance, Fiorentina would hold a notable advantage over Fulham: Mastantuono also holds Italian citizenship, meaning he would not occupy one of Fiorentina's two permitted non-EU registration slots in Serie A - a meaningful logistical edge given how tightly clubs in Italy have to manage those spots.
Fulham's interest, revealed earlier by AS, would also take the form of a straightforward season-long loan. For Real Madrid, that structure is the only kind of move under consideration regardless of destination - the club has no intention of relinquishing control over a player it views as still developing, having paid €63.2million for him last summer and considering him, at 18, far from the finished article.
Nothing has been decided as things stand. Mastantuono remains a Real Madrid player and continues training under Jose Mourinho, but the level of competition for places out wide in Madrid's attack suggests his path to regular minutes this season may lie elsewhere.
Real Madrid face Fiorentina in their first official pre-season friendly on 1 August, a fixture that could yet play a part in accelerating any decision on the Argentine's immediate future.
Mastantuono arrived at the Bernabeu last summer in a deal that made him the most expensive sale in Argentine football history, with Real Madrid triggering his release clause at River Plate and the total package - including taxes and add-ons - reaching the €63.2million figure.
He signed a six-year contract understood to include a release clause in excess of €1billion, reflecting the value the club places on protecting one of its most closely guarded young assets even as it explores ways to get him regular football elsewhere in the short term.