The season in Spain will restart on June 8 with the coronavirus lockdown now easing.
"Hazard is back and with a lot of energy," Courtois told El Mundo.
"The stoppage has enabled him to get fit again. He can help us win the title. He is a key player for us."
Courtois continued: "I've known him for nine years and he has always wanted to play here - it was his dream. When we were both at Chelsea we spoke about the possibility of ending up playing here.
"'La casa blanca! La casa blanca!' (white house) we would say. It was nice to imagine it and now we really want to win a lot of trophies."