The Spanish giants paid €45 million (£39m/$52m) for the 17-year-old, but he has yet to begin training alongside Jude Bellingham and the rest of the squad.
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Mastantuono is only permitted to train with Real Madrid after turning 18, meaning he must train alone until his birthday on August 14 and is not allowed to join Xabi Alonso’s squad.
During this period, the club will closely monitor his progress.
Real Madrid will begin the new La Liga campaign on August 19 at home to Osasuna.