The Spanish champions are determined to reinforce their Castilla squad and have apparently secured the services of the young Brazilian full-back
The 18-year-old defender impressed in Fluminense's youth ranks earlier in the year after joining the Rio de Janeiro-based club from Paulinas de Brasil
His performances encouraged Gestifute, the company owned by Fifa agent Jorge Mendes, to secure ownership of the player before sending him to Rio Ave in June to get more games under his belt.
However, the Brazil Under-20 international has now been shipped out on loan to Madrid's Castilla squad, having already taken part in the club's training session on Thursday.
Fabinho will fill in for Dani Carvajal, who signed for Bayer Leverkusen earlier in July.
Meanwhile, Spanish news daily AS reports that Los Blancos are also keen to sign Cordoba's 21-year-old winger Borga Garcia and Karpaty's 23-year-old midfielder Lucas Perez, both of whom will join the Castilla squad.