That is according to former Real Madrid president Ramon Calderon.
Bale looks set to leave the Bernabeu this summer following strong comments from Zidane.
The France legend said of Bale: “We hope he leaves soon. It would be best for everyone. We are working on his transfer to a new team.
“I have nothing personal against him, but there comes a time where things are done because they must be done.”
Calderon has revealed Zidane wanted Bale to be sold in the summer of 2018 and instead asked for Cristiano Ronaldo to stay.
However, Ronaldo ended up joining Juventus and Bale remained a Real player, with Zidane resigning from his role.
Zidane returned as Real manager in March following the sacking of Santiago Solari.
And Calderon claims he was told Gareth Bale would be sold as soon as possible.
“I think it’s unsustainable [for Bale to stay at Real] really, for sure,” he told Sky Sports News.
“For everyone, for the player, for the coach and for the club.
“Zidane discounted him a long time ago when he left the club at the beginning of last season.
“He said he wanted to keep Cristiano and to sell Bale.
“The president didn’t follow that advice, so that’s why [Zidane] left the club.
“When he came back two months ago he was promised Bale won’t be there in the future.
“Maybe his comments weren’t really appropriate but what he has showed is he’s fed up with the situation and the best thing to do is find a solution.”