France under-20 international Hernandez, 19, has been with Atletico's youth system since 2008 and, despite never playing for the first team, has established himself in Spain's top flight while on loan at Alaves last season.
The defender, whose brother Lucas plays for Atletico and whose father Jean-Francois played for a string of clubs including Atletico, gained admiration for his explosive bursts down the left flank with Alaves last campaign.
He played 32 league games and made six appearances in the King's Cup, scoring a scorching free kick in their 3-1 defeat to Barcelona in the final at the Vicente Calderon, his only competitive appearance in Atletico's stadium.
News of the player's imminent departure from Atletico to Real leaked in the Spanish media last month on the eve of the Champions League semi-final second leg between the two clubs in May, cranking up ill feeling after a 17-year spell in which no player had directly joined one club from the other.
The move is expected to be confirmed next week, after Real president Florentino Perez renews his mandate until 2021, with no candidate declaring their intention to take him on two days before Sunday's deadline.
The last player to move immediately from Atletico to Real was former Argentina midfielder Santiago Solari in 2000, after Atletico had been relegated from Spain's top flight.
Former Mexico striker Hugo Sanchez is one of the most famous players to have crossed the divide, joining Real from Atletico in 1985 and helping them to five consecutive La Liga titles, while Bernd Schuster went the other way in 1990.
The thought of Atletico losing one of their hottest prospects in Hernandez to their rivals will also bring back painful memories of how Raul Gonzalez left their youth set-up as a teenager when it was dissolved in 1992, leaving the striker to join Real, where he went on to score 323 goals.
(Reporting by Richard Martin; editing by Andrew Roche)
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