Bale had hoped to feature in their Champions League tie with Napoli.
Zidane said: "This is his first week back training with us, he's in good shape physically.
"You'll have to see whether he's in the squad or not, or whether he'll be in on the weekend. We'll see. He's had almost three months out and we have to take it slowly with Gareth. The important thing is that he's back training with us."