Real Madrid have stuttered in LaLiga, with three consecutive draws seeing them slip to second behind city rivals Atletico Madrid.
Ronaldo has only played four of seven league games after a delayed start to his season due to a knee injury, with just a goal and an assist in those outings.
Capello feels the Portugal captain’s lack of fitness is what is costing Madrid.
“The problem for Real Madrid now is Cristiano Ronaldo,” he told Onda Cero.
“Because he is not well physically and is their best player.”
Atletico and Madrid are locked on 15 points atop a congested table, with Sevilla (14), Barcelona and Villarreal (both 13) not far behind.