Courtois has struggled for form and fitness this season.
"It wasn't true that he had anxiety," Martinez said of the reports that came out following the Club Brugge match.
"He is still feeling the effects of a stomach bug and needs to regain a kilogram or two.
"That's what can happen with a stomach bug.
"He is a player who dreamed of playing for Real Madrid from a young age and he is ready to define an era there.
"I am convinced that he is the best goalkeeper in the world."