The club said in a statement the Belgian international, who was substituted at half-time in Tuesday's last 16 tie against Manchester City, had been diagnosed with a muscular injury "to the right quadriceps".
Real did not specify how long the thigh injury would sideline him but sports daily AS said Courtois, 33, could be out for six weeks, which would cause him to miss the quarter-final matches against Bayern on April 7th and April 15th.
His likely replacement is the Ukrainian Andriy Lunin.