The Blancos have confirmed that he will be out for at least three weeks with a calf muscle issue, which would see him return to fitness in the middle of May.
And therefore Bale, 27, will be absent in both legs of Madrid's last four clash against fierce rivals Atletico, in what is a bitter blow after a difficult weekend for Zinedine Zidane's men following their narrow 3-2 defeat against title rivals Barcelona.
A statement on Madrid's website read: “Following the tests carried out on Gareth Bale by the Sanitas Medical Services, the player has been diagnosed with a grade 2 injury in his left calf.
“His recovery will continue to be assessed.”
Bale was handed a starting berth by Zidane in Sunday's Clasico but was replaced by Marco Asensio in the first half after sustaining the problem.
However, speaking to the media ahead of Wednesday's clash with Deportivo La Coruna, Zidane defended his decision.
"Bale was 100 per cent,” said Zidane. “He told me he was at 100 per cent, that the two training sessions before the game were perfect. We cannot avoid this happening.
"Injuries happen, it is part of a player’s life, nobody can do anything about that.
"I hope he is back before the end of the season. It's grade two [the severity of the injury], yes. But the doctors tell me it's a small thing. I trust the staff here."
The injury comes as the latest setback in a campaign blighted by fitness issues for Bale, with the Wales star having missed a large chunk of winter period with an ankle problem.
He returned in March but was soon out of action after being sent off in Madrid's heated 3-3 draw with Las Palmas in La Liga.