The Welshman, who missed Saturday's win at Levante, has already been ruled out of their Champions League tie at Liverpool.
He suffered the setback in training on Friday during shooting practice.
Now, Real have announced that Bale "has a muscular lesion in the right piriformis muscle" - medical experts say the strain usually takes three weeks to recover from.
That period can be cut with intensive rehab, but it will still mean Bale missing the Barca clash this weekend.
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