The Welshman is believed to be surplus to requirements at the Bernabeu, and multiple reports have suggested that the Red Devils are one of several clubs keen on his services.
A string of injuries has meant that Bale has played just 10 times for club and country this season, but Zidane continues to back the 28-year-old.
"We know what Gareth can give us. Many things," the Frenchman is quoted by The Sun as saying. "He is very strong, fast, technically very good, works very hard.
"It's true there has been a lot of talk about his injuries, and that he plays just three games before another [absence].
"He's an important player for us and he knows that. What he can give us is strength. He's fast; he can open things up for us and can give us a lot of things.
"I don't know when he is going to play. He hasn't had any trouble today, but we'll see when he can play."
Bale has played 159 times for Real since his 2013 move from Spurs, scoring 70 goals and registering 55 assists in the Blancos shirt.