Ronaldo ended 2014 in victorious fashion, helping Real to Club World Cup success, before then winning the FIFA Ballon d'Or ahead of Lionel Messi and Manuel Neuer, but he has struggled for form since returning from La Liga's winter break.
The Portugal star has scored just four goals in eight appearances across all competitions since the turn of the year, compared to the 10 in nine outings he managed for Real at the end of 2014.
And Capello finds Ronaldo's unimpressive run somewhat concerning, suggesting Real give the 30-year-old a medical examination.
"It is very important that he is not at his usual level," Capello told Onda Cero.
"At the end of last year Madrid began the game already 1-0 ahead, because they had Cristiano.
"There should be a talk with the doctors to know how each player is, including looking at their blood tests.
"But he is an extraordinary player and he will return to 100 per cent very soon."
Capello also commented on the return to fitness of Pepe after a month out with a rib injury, insisting Real's defensive issues should begin to improve with the centre-back's re-emergence.
The Italian added: "When you have a defence which helps you and gives you security, you do not make so many mistakes.
"Madrid at the moment are not so fresh, but when their centre-backs return, they will get many other things back.
"When [Raphael] Varane plays with [Sergio] Ramos or Pepe, he is a different player, he has more confidence.
"When the biggest games come, Madrid need their players with most character and quality."
