Bale left out as Madrid aim to halt Maradona-backed Napoli
Bale returned to training for the first time since suffering ankle ligament damage in November on Sunday.
Bale returned to training for the first time since suffering ankle ligament damage in November on Sunday and has been left out of Zidane's 19-man squad.
Cristiano Ronaldo has been passed fit, though, as the Portuguese has shrugged off a knock picked up in Saturday's 3-1 win at Osasuna to keep Los Blancos top of La Liga.
"He has had nearly three months out and we have to go slowly with Gareth," said Zidane on Tuesday.
"Everyone wants to win the Champions League, but it is a hard and long road and we are on that path.
"The good thing is that we are in good shape. We have everyone fit, that means it will be harder to pick the squad, but I'm happy to see everyone ready and committed for the return of the Champions League."
Napoli's sensational free-scoring form that has seen them reel off an 18-game unbeaten run has seen the Italians' hopes of shocking the holders soar.
Among the expected 10,000 strong travelling support will be Napoli's greatest ever player as Maradona arrived in Madrid on Monday.
"Thankfully he is not playing! We are happy about that," said Madrid midfielder Luka Modric on the presence of Maradona.
"He is one of the most important players in the history of football. It is nice he is here to see the game live."
Zidane, though, insisted he is more concerned by the likes of the on-fire Dries Mertens, who has scored 14 goals in his last 11 outings, including three hat-tricks.
"He (Maradona) was one of the best players in the history of football, but I am more worried about those that are on the field."
No side has managed to retain the competition in the Champions League era with AC Milan 27 years ago the last team to win back-to-back European Cups.
However, Modric believes the 11-time winners are better placed than anyone else to bring that streak to an end in June's final in Bale's home city of Cardiff.
"We are Real Madrid and it has never happened, but we think we can be the first team to defend the title," added the Croatian.
"Tomorrow is the first step against Napoli, that is the most important thing not looking further ahead. We want to be in Cardiff but we need to go game-by-game."
Napoli have a returning star of their own as Arkadiusz Milik has made the bench for victories over Bologna and Genoa after suffering cruciate ligament damage in October.
However, the Polish international will start on the sidelines again with Mertens partnered by Lorenzo Insigne and former Madrid winger Jose Maria Callejon.
"We are going to play an away match against the champions of the world and a really strong side," Napoli boss Maurizio Sarri told the club's website.
"They are so strong that we don't even know if we will be able to stop them.
"I don't know what to expect and what it will happen. I just want to say that I want commitment, dedication and nerve in order to understand exactly who we are and where we can get to."
Napoli will be forced to abandon one lucky charm, though, as Madrid will be the ones in their famous white jerseys on Wednesday.
The turnaround in Napoli's fortunes in recent months coincided with superstitious president Aurelio De Laurentis's decision to replace the club's traditional sky blue colours for white back in November.