The Portuguese attacker made the difference for the Blancos in a hostile environment, prompting the assistant coach to hail his performance.
The Portugal international received a hostile welcome at San Mames, but silenced the home crowd with a fine performance as he scored twice and created his side's third goal.
"He did what he had to do and scored two goals that won us the match. He has once again shown the type of player he is," Karanka said at a press conference. "Football is for men and even more so at San Mames. Athletic's players did what they needed to do. Cristiano had the match he had and scored twice.
"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."
Ronaldo has already netted 31 goals in 30 Liga appearances so far this term.