The 18 year-old has joined Real on a six-and-a-half year deal after Fla accepted Real's €30m offer for his transfer before Christmas.
Reinier helped Fla win the Brasileiro and Copa Libertadores last season in Brazil, while he also was part of Jorge Jesus' squad which reached the Club World Cup final before their defeat to Liverpool in Qatar last month.
The attacker has now committed to Real, though Castilla coach Raul admits they'll have to wait to see him up close due to a pre Olympics tournament involving Brazil.
Quoted by Le 10 Sport, Raul remarked, "We have to wait until the end of the Olympics and wish him the best for the moment. When he arrives here, we will see under what conditions he will find himself."
While it's been suggested Reinier could be added to Zinedine Zidane's first team squad, Raul intimated he expected the new signing to be registered with Castilla.
The Los Merengues legend continued: "He is a player with a lot of potential, a lot of future and he will bring us a lot.
"Competition will be strong in this position. It is never easy to play in South America and come to play in Europe. He is a very talented player, but there will be complicated matches."
Raul oversaw Castilla's 3-1 win against Segunda B opponents Pena Deportivo on Saturday with Miguel Baeza Perez netting a hat-trick. Reinier will be a welcome addition to the Castilla squad as they currently sit tenth on the Segunda B table, nineteen points away from leaders Atletico Madrid B.