The Portuguese forward missed training on Monday after picking up a minor injury in Saturday's La Liga win at Osasuna.
However, Wales international Gareth Bale (ankle) will not play.
Striker Arkadiusz Milik is fit for Napoli.
The Poland international has recovered from a cruciate ligament injury and has been included in the Serie A side's squad. Defender Lorenzo Tonelli, who has not played since 21 January, has also travelled.
Holders Real Madrid are aiming to reach the Champions League quarter-finals for a seventh successive year.
Napoli, meanwhile, are only in the last 16 for the second time but Real coach Zinedine Zidane is not taking anything for granted against the Italians.
"You can't win a knockout tie in one match," he said. "We're going step by step and game by game.
"Everyone wants to win the Champions League but it's a long, hard road we're on.
"Napoli are a good attacking side and they play with a lot of intensity and speed. They're small, tricky players with good technique.
"We have the tools to do them harm and we want to do that."
La Liga leaders Real have lost just two games all season - against Sevilla in the league on 15 January and against Celta Vigo in the Copa del Rey three days later.
Napoli, meanwhile, go into the game in impressive form, having not lost any of their 18 games since October.
"With the awareness that we are going to play an away match against the champions of the world and a really strong side, we will be a bit worried," said Napoli coach Maurizio Sarri.
"Sometimes it's a positive feeling because it gives you a boost. On the other hand if you don't have courage to face your opponent with determination there is no need to play."
Cristiano Ronaldo has recovered from a knock to be included in Real Madrid's squad to face Napoli in the first leg of their Champions League last-16 tie at the Bernabeu on Wednesday.