Last night's 0-0 draw at Liverpool saw Real reach the final four 3-1 on aggregate.
Valdano later said: "In the first five minutes, Madrid lived supported by Courtois and an error by (Mohamed) Salah, which was not good enough for his class, but he (Courtois) did a very professional job.
"People with a lot of experience have worked under the law of minimum risk. The Valdebebas game and his enormous effort was the difference."
He added: "Courtois is a great team goalkeeper. In moments of great demand, his interventions have kept the team in the game. Between him, the two centrals and Casemiro, who grew hugely, they supported Madrid."