The 24-year-old has suffered a stop-start beginning to his Blancos career and will miss the Liga side's latest European tie.
The 24-year-old has had a frustrating start to his Madrid career since his move from Tottenham this summer, with another thigh problem ruling him out of September's clash with Getafe.
"After the tests carried out today [Wednesday], the player has been diagnosed with a muscle contracture in his left thigh," Real Madrid said on its official website.
"It remains to be seen how the condition develops."
Bale has yet to complete a full 90 minutes for Madrid and has scored once in his three appearances for the club.