Mastantuono joined from River Plate for €45 million last summer and made a bright start before a groin injury in November disrupted his rhythm and cost him his place in the starting lineup.
He has managed only nine La Liga starts this season, with his role shrinking to cameos off the bench in recent weeks. He featured for just a few minutes in Wednesday's 2-1 win over Alavés.
According to Marca, Real Madrid remains fully convinced by Mastantuono's long-term potential but believe he needs a less demanding environment to recover his confidence and accumulate regular minutes.
A loan with no purchase option is the preferred structure — consistent with how the club has managed other young talents, including Endrick, who joined Lyon on loan in January after a similar period of under-use.
Como have been discussed as a potential destination, according to Fabrizio Romano, in a deal that would be linked to the return of Nico Paz. El Chiringuito reports that Mastantuono is prepared to accept whatever the club decides.