The 20-year-old has returned to the Spanish capital for preseason after scoring eight goals on loan at Lyon in the 2025/26 campaign.
However, following the summer arrivals of Yan Diomande and Carlos Espi, the Brazil international was linked with another loan exit amid links to Liverpool.
The club have since opted against letting him go, but Mourinho is concerned by a fresh injury setback, and admitted he will have to speak with former Los Blancos boss Carlo Ancelotti over his availably for the Samba Boys next month.
"He has a problem with the muscle and it's affecting a tendon. He's a player I like a lot and he's had a lot of contact from other clubs. I wanted him to stay and that's why I haven't loaned him out or sold him.
"I'm not sure he'll be in Ancelotti's first squad of the season, but Carlo is my friend, so I'm sure he'll call me directly."
Endrick was not included in Ancelotti's squad for the 2026 FFA World Cup and Mourinho will be hoping he's omitted from the incoming friendlies against Australia and India in those two countries in September/October.