The Portugal international started the 2014-15 campaign in sublime form and had already netted 25 goals after just 14 Liga appearances.
However, Ronaldo has been struggling to replicate that impressive form in 2015 and has now gone three matches without a goal in the Primera Division.
Consequently, Capello is urging Madrid to keep a close eye on their star player's fitness.
"It is an important matter that Ronaldo is not at his usual level," Capello told Al Primer Toque.
"Toward the end of 2014, Madrid were already a goal up before the game even started because they had Ronaldo. This is no longer the case.
"Madrid's medical staff should keep a close eye on him. He does not look 100 per cent match fit. Madrid should find out what the problem is, because this is affecting the team.
"Madrid should be talking with their medical staff to discuss how each of their players are doing, including looking at blood tests.
"But Ronaldo is still an extraordinary player and I am confident he will get back to his usual best very soon."
Ronaldo was being plagued by physical problems toward the end of the 2013-14 campaign, but played through the pain barrier in the Champions League final, before travelling to Brazil with Portugal for the 2014 World Cup.
The 30-year-old has scored 36 goals in 33 appearances for Madrid in all competitions so far in 2014-15.
Despite his current drought, the prolific attacker still holds a two-goal lead over Barcelona star Lionel Messi in the race for the European Golden Shoe.