Navas, whose last game was the Champions League final triumph on penalties over Atletico Madrid on May 28, required surgery to resolve chronic tendinitis in his left foot.
"Keylor is back with us and in the squad," Zidane told reporters on Friday. "He is a key player for us.
"Kiko (Casilla) has done very well and I will talk to the two of them to see what we can organise (playing time)."
Navas missed the Copa America with Costa Rica and Real's opening five La Liga games as well as the 2-1 triumph over Sporting Lisbon in their Champions League group opener.
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Real, unbeaten and two points clear of Sevilla at the top of La Liga, missed out on a league record 17th successive win when they drew 1-1 with Villarreal on Wednesday.
Left back Marcelo and midfielder Casemiro have joined defender Pepe on the sidelines due to injuries.
Marcelo has a calf muscle strain while Casemiro has fractured a bone in his leg.
"Losing two starters is a concern but we can't do anything about it," Zidane said. "We don't have a player that plays like Casemiro. It will be different but it's not a problem.
"We have a big squad and we will make changes."
Champions Barcelona, three points adrift of Real, have lost forward Lionel Messi for three weeks due a groin injury.
"Barca are worse off without Messi," Zidane said. "Messi is Messi and we know what he brings to the team. It will be different."
(Reporting by Adriana Garcia, editing by Ed Osmond)
