Bale has not played since Jan. 17 after damaging a calf muscle in a La Liga match against Sporting Gijon.
Spanish media have reported that Bale, whose Real career has been blighted by a succession of injuries, suffered a setback in his recovery.
Zidane said Real needed to be "calm" about an injury that has been "going on for a bit more time for him".
"If he's uncomfortable and it hurts him, it's normal that he could have a setback," Zidane told a news conference on Friday.
"He's a very important player for us and we're not going to take any risks. We want him at 100 percent, not at 80 or 85."
Zidane said Bale was "in good spirits" although he is set to miss at least the first leg of Real's Champions League last-16 clash with AS Roma in Italy on Wednesday.
"Every player has a different recovery process and he has suffered with his pains. There's no need to keep going over it," the Frenchman said. "We have to give him time."
Real, third in La Liga, four points behind leaders Barcelona, host sixth-placed Athletic Bilbao on Saturday (1500).
Zidane said Croatian midfielder Mateo Kovacic would start in place of Isco while defender Marcelo, who injured his shoulder earlier this month, remains sidelined.
(Reporting by Justin Palmer; editing by Toby Davis)
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