Courtois is believed to have suffered a hip injury in training this week, with tests subsequently revealing the extent of the damage. Real have offered no estimated timeframe on his recovery, other than stating it will be 'monitored'.
Navas had been due to return to the Real lineup this week for Copa del Rey action against Leganes, but there may now be an opportunity to keep that place for Sunday's La Liga clash with Real Betis.
The Costa Rican international won three Champions League titles in his four seasons as Real's starting goalkeeper after replacing Iker Casillas between the sticks, only to be benched this season after Real landed World Cup Golden Glove winner Courtois.
Navas played in Real's first two La Liga fixtures of the season back in August as a result of Courtois' prolonged involvement in the World Cup and delayed summer break, as well as initially retaining his place as the club's preferred Champions League goalkeeper.
Navas, who was briefly linked to Arsenal earlier this month until it was rumoured that he has bizarrely agreed to extend his contract at the Bernabeu, wasn't even given a look in as Real travelled to the FIFA Club World Cup in later December.