"Following tests carried out today on our player Thibaut Courtois by Real Madrid's medical staff, he has been diagnosed with a muscle injury to the rectus femoris of his right quadriceps. His condition is being monitored," the club said in a statement on Thursday.
The injury compounds Real Madrid's mounting fitness concerns, with key players Kylian Mbappe, Jude Bellingham, Rodrygo and Eder Militao already struggling with injuries.
Ukraine goalkeeper Andriy Lunin will step into the breach, starting with Sunday's derby against Atletico Madrid in LaLiga.