Mastantuono joined Real Madrid from River Plate last summer for around €45m and made 35 appearances in all competitions in a debut season constrained by intense competition for attacking positions, contributing three goals and one assist in fewer than 1,500 minutes.
Mourinho has effectively sanctioned a temporary exit to allow the attacking midfielder to develop elsewhere, with Madrid seeking a club that can offer Champions League football and contribute toward his €7.3m annual salary.
That condition represents a significant obstacle for Tottenham, who failed to qualify for the Champions League last season.
According to Spanish outlet Sport, Tottenham face competition from eight other clubs: Villarreal, Porto, Sporting CP, Benfica, Inter, Juventus, River Plate and Rennes — with Villarreal currently the frontrunners given their European pedigree.
Tottenham had previously been rebuffed in attempts to sign Mastantuono permanently, but are now open to the loan structure Madrid are proposing.
The Argentine fits the creative profile new manager Roberto De Zerbi wants to add to his squad, though questions remain about whether Spurs can guarantee the regular minutes Madrid will demand given Xavi Simons, James Maddison and Dejan Kulusevski already occupy similar positions.
Real Madrid will not sanction a permanent transfer and will include no purchase option in any loan agreement. Mastantuono remains contracted to the Bernabéu until 2031.
The number of clubs circling reflects his status as one of world football's most coveted teenage talents. Whoever lands the loan will be doing so knowing Madrid retain full control over his long-term future.