Bale, 30, has won four Champions League titles since joining Madrid from Tottenham Hotspur in 2013 for a then world record 100 million euros ($112.20 million); he struggled to hold down his place in the starting side last season.
Amid suggestions that competition will only increase next season after the arrival of Belgian Eden Hazard, Bale was left out of Saturday's 3-1 defeat by Bayern Munich in a pre-season clash in Houston.
"He wasn't included in the squad because the club is working on his departure ... that's why he didn't play," Zidane explained. "We'll have to see if it goes through tomorrow, if it does then all the better. Let's hope, for everyone's sake, that it happens soon."
Zidane's comments drew a stinging reply from Bale's agent Jonathan Barnett, who labelled the Frenchman "a disgrace". "Zidane is a disgrace to speak like that about someone who has done so much for Real." Barnett was quoted as saying by ESPN. "If and when Gareth goes it will be because it is in the best interest of Gareth and nothing to do with Zidane pushing."
Bale was criticised by the Spanish media last season after failing to fill the void left by Cristiano Ronaldo's departure to Juventus as Madrid came third in La Liga last season, 19 points behind champions Barcelona. Zidane, however, said there was nothing personal in his decision to deem Bale surplus to requirements.
"There comes a time when things are done because they need to be done. I've not got anything against him. We have to make decisions and change things, that's all there is to it. I don't know if this will happen in 24 or 48 hours ... it's for the best for everyone."
Bale's high wages, reported to be 600,000 pounds per week, mean few clubs would be capable of signing him permanently. Barnett last month ruled out chances of him leaving on loan, saying he was happy to stay amid reports linking him with Manchester United, Bayern Munich and Spurs.
