Zidane had previously earmarked the second leg of the last-16 tie with Napoli as the likely time for Bale to make his comeback, before footage of the forward looking sharp on the pitches at Valdebebas encouraged speculation that he could play at the Santiago Bernabeu this week.
Although Zidane acknowledged Bale has looked in good shape in recent days, he remains uneasy about the prospect of rushing him back after close to three months without a game.
"It's the first week that he's trained with the group," he told a news conference. "He's good, he's good physically and mentally, which is the most important thing.
"At the same time, after two or three months out, we have to be very careful with Gareth."
Team-mate Luka Modric praised Bale for his efforts to return to fitness early and says he is mentally primed to get back into action.
"It's always difficult when you don't play because of injury," said the Croatia international. "I think he has recovered well. He looks fired up to come back as soon as possible.
"We miss him and we want him to be back. He's strong and this doesn't affect him mentally. He's recovered in the best possible way."