By FOX Sports Staff
The Portuguese attacker had picked up an injury late in Real's 3-2 Champions League defeat to Galatasaray in Turkey, and sat out Thursday's training session.
Ronaldo did light training on Friday and got the nod from manager Jose Mourinho to start Sunday's La Liga game at the hostile San Mames.
The former Manchester United star repaid his manager's faith by scoring twice as Real cruised to a comfortable 3-0 win over Bilbao.
"Football is for men and even more in San Mames," Karanka was quoted as saying on the club's website.
He added: "Athletic's players did what they needed to do. Cristiano did well in a tough atmosphere and has once again shown the type of player he is. He did what he had to do and scored two goals that won us the match".
Real took the lead in only the second minute after Ander Herrera brought down Ronaldo, who punished the home side with a delightful curling free-kick from 30 yards.
Real doubled their lead on 69 minutes when Ronaldo nodded home a free-kick from Xabi Alonso before the Portugal international set up Gonzalo Higuain for the third and final goal.
"He is dedicated to playing and scoring goals, that is what he does, and that is what he benefits from and that is that," Karanka said.
Ronaldo has 31 league goals this season for Madrid.