Hazard appeared to be caught on his left knee by an Alaves defender as he chased down the rebound after his shot was saved, and was pictured grimacing moments later.
"Eden, I think and I hope it was just a simple knock. That's what he told us. That it's just a knock, that it's not muscular," Zidane said after the 2-1 loss.
The 29-year-old was limited to just 22 appearances in 2019-20 after joining Real from Chelsea for 100 million euros ($119.6 million), scoring just one goal.
He missed the start of this season due to injury and then tested positive for coronavirus on November 7.
He scored a penalty for his first Champions League goal in a Real shirt in the club's 2-0 win at Inter Milan in midweek.