The Belgian shot stopper is set to have tests on his left leg on Tuesday to discover how serious his injury is, according to Spanish outlet AS, who say he must wait an extra day to be looked at ‘due to inflammation’ in the area.
Madrid next play against at home against Eibar on Friday night before travelling to the Etihad to face City, and he will be a question mark for both games.
Real Madrid will travel to the Etihad next Tuesday chasing a 2-1 deficit in the second leg, and being without their No 1 keeper will come as a further blow after they were outclassed at the Bernabeu.
Los Blancos took the lead in the first leg through Isco before goals from Gabriel Jesus and Kevin De Bruyne turned the tie on its head.
The No 1 contender to replace Courtois between the sticks would be Alphonse Areola, who filled in for the ex-Chelsea star in February for a 4-3 defeat by Real Sociedad.