Courtois was signed from Chelsea after the 2018 FIFA World Cup despite Navas winning UEFA's best goalkeeper award after helping Madrid win the UEFA Champions League for three consecutive seasons.
Santiago Solari then put faith in Courtois during his four months in charge but Navas, a favourite of Zidane's, came back in for the Frenchman's opening game against Celta Vigo last month.
"At the moment there are three good keepers and we are going to finish the season with them," Zidane said, as per AFP.
"But I assure you that next season there will be no debate about the goalkeeper, I'll tell you that now. It's going to be very clear."