Zinedine Zidane's son played for the first team against Huesca on Sunday.
At Valdebebas he worked closely with Casilla before the Spaniard left for Leeds United in January.
"Luca deserves all the good that comes his way," Casilla told MARCA.
"I'll be the first to be happy for him because he's a great person and a great goalkeeper.
"I think he's made a leap with his maturity this year and he's doing well with Castilla.
"He has that thing that a goalkeeper needs."