The 31-year-old has scored just three times in 20 appearances for Los Blancos this season, while he has only featured twice for Zinedine Zidane's side since their campaign resumed in June.
Bale has also not been included in the Madrid squad for Friday's Champions League last-16 second leg against Manchester City despite being fit and available for selection.
According to Montero, the Wales international is keen on a return to England, where he previously represented Southampton and Tottenham Hotspur, but a move is unlikely due to the attacker's substantial wages.
"One year ago, I was in Washington with Zidane, the day he said it's better for him [Bale] to go today, better than tomorrow," Montero told Sky Sports News. "The truth is there was only one option - China [Jiangsu Suning made an offer for Bale last summer]. He didn't want to go to China.
"He wants to return to the Premier League, but he earns €70m (£63m) [in a five-year contract] at Real Madrid.
"There's no club in the world, after a player that has been two seasons without playing more or less, who can pay for that. It's a lot of money. So, I think it's going to be a tricky situation but I am afraid he's going to stay at Real Madrid."
Tottenham and Manchester United have both recently been linked with a summer move.