Kepa is now on-loan at Real to cover Courtois after knee surgery ruled him out for the season.
Courtois said: “I think Chelsea made a mistake with this guy.
"He is an excellent goalkeeper who is very strong. I have no doubt that he is one of the top five best goalkeepers in the world at the moment."
This season, Kepa has played five matches for Real Madrid in which he's conceded five goals.