The 24-year-old has had a frustrating start to his Madrid career since his €100m move from Tottenham this summer, with another thigh problem ruling him out of September's clash with Getafe.
And los Blancos confirmed news of the Wales international's latest injury setback on their official website on Wednesday.
"After the tests carried out today [Wednesday], the player has been diagnosed with a muscle contracture in his left thigh," Real Madrid said.
"It remains to be seen how the condition develops."
Bale is yet to complete a full 90 minutes for Madrid and has scored once in his three appearances for the club.