Earlier this week, former Real Madrid boss Capello said that Los Blancos have drawn their last four games in all competitions because Ronaldo is currently struggling to find his best form.
Santos, however, has leapt to the defence of his captain ahead of Friday's 2018 World Cup qualifier at home to Andorra, where Ronaldo will earn his 134th Portugal cap.
"I respect Capello a lot, but honestly it would mean more if those comments came from Zidane. When you have the best player in the world, it becomes difficult, Cristiano is Cristiano - he always want to be available for Portugal," Santos told reporters.
"He carries great value, especially with the younger players and that's very important in a team where many players are young. He has won many titles, but he always wants to win more and more - a lot of our team have won a title, but do not have many career titles.
"It's so important to have a winner in the team, as it guides the others - his presence gives everybody else strength and he guides them."
Ronaldo has only scored one league goal for Real Madrid this season.