Among them is Lamini Fati, widely regarded as one of the most promising defenders coming through Valdebebas.
The 20-year-old Spaniard is using the opportunity to stake a claim as the academy's go-to option should Madrid need defensive reinforcements from within during the season ahead. Fati made 32 appearances in his first year with Castilla and now gets his first taste of pre-season alongside the first team - a chance many young players dream of, and one that could go a long way toward winning over the new head coach.
Standing at 1.84m, he has stood out since a young age for his physicality, defensive versatility and comfort on the ball, and was already called into the first-team squad for last season's fixture against Celta Vigo in matchday 27.
Fati is not the academy's only central defender with first-team ambitions. Joan Martinez, 18 and standing 1.93m, offers a similarly imposing physical profile allied to strong game intelligence, and the pair form a defensive partnership of a clearly higher standard than the level at which Castilla currently competes, Primera RFEF.
Comparative data shows Martinez winning more aerial duels and making more interceptions, while Fati holds an edge across most other defensive metrics. Crucially for his prospects, Fati is left-footed and operates naturally on the left side of a central pairing - a profile the first-team squad currently lacks entirely, since Madrid have no left-footed centre-back on their books, a gap that works in the young defender's favour.
With the World Cup contingent still to return, Fati has a genuine window to impress in the club's preparation matches. The key test comes on 28 July, in a behind-closed-doors friendly against Leganes, where Mourinho will be taking notes on his 16 academy players; the closed-doors format should allow the youngsters to play with less pressure against senior opposition than they are used to facing.
A strong showing there could open the door to further involvement across the rest of Madrid's pre-season schedule, which continues with fixtures against Fiorentina on 1 August, Deportivo de la Coruna on 12 August and Schalke 04 on 16 August.
Once pre-season concludes, Fati will return to Castilla, who are chasing a target the club has pursued since dropping into Segunda B - now Primera RFEF - back in 2014: promotion to the Segunda Division, which would allow Real Madrid to develop its young players in a more competitive environment.
With that goal in mind, Fati will be giving his all in every fixture in the hope of one day earning the call from Mourinho and fulfilling the dream shared by so many academy players: a debut for the first team.