Eden Hazard, 29, faces a lengthy spell on the sidelines after sustaining the injury in Saturday 1-0 La Liga defeat at Levante.
The Belgian had been out of action for nearly three months due to another ankle injury and was making only his second appearance since his return. "I don't know if his season is over, I don't know but I hope not," Zidane told a news conference before Wednesday's Champions League last 16 first-leg clash against Manchester City.
Hazard was seen as the heir to Cristiano Ronaldo when he joined from Chelsea last year for 100 million euros ($108.68 million) but injuries have limited the Belgian to 15 appearances and one goal.
"I don't know if he'll need an operation, it's not my area. I can see he's not happy. It's important he stays positive but, of course, it's difficult," Zidane added. "It's a bad moment, it won't be easy and he'll feel like this for a while."
