Saul, 22, spent two years with Real between 2006 and 2008 before joining Atletico, where he has developed into a key player over the last few seasons.
The Spain international will be in Atletico's starting XI for Saturday's Madrid derby at the Wanda Metropolitano, and he has reflected on the time that he spent with Los Blancos.
"Reflecting back, my time at Madrid was positive for allowing me to grow and mature," Saul told Movistar+. "Now I realise that it was ultimately a good thing for me as it allowed me to reach the level I'm at now, even if it didn't feel good at the time."
Saul has scored once in 11 La Liga appearances for Atletico this season.