Ronaldo missed last Saturday's 4-1 LaLiga win at Eibar due to what the club described as "muscular problems".
His return was expected after he took part in a recovery training session with the squad on Sunday and he has now been named in the 22-man party for the trip to Naples.
Fellow forwards Gareth Bale and Alvaro Morata also return for the game in Italy after missing the Eibar victory through suspension.
Holders Madrid lead the last-16 tie 3-1 after the first leg at Santiago Bernabeu last month.
Raphael Varane and Fabio Coentrao are the club's only injured first-team players.