Wales star Lee Byrne broke a bone in his back during Clermont Auvergne's Heineken Cup victory over Leinster on Saturday.
The injury is not career-threatening, but a statement on the Welsh Rugby Union website said he is expected to miss the Six Nations.
Clermont, who lead Leinster by eight points in their pool with two games remaining, said that they would know more about the length of time he would be absent once the 32-year-old undergoes further tests.
"He suffered a broken bone in his fifth vertebra," the club said on their website.
"The length of his unavailability will be ascertained in the coming days by complementary examinations but we will not be seeing Lee Byrne again until at least the beginning of 2013."
Can Leinster still defend their European crown after Saturday's loss to Clermont?
Yes, they'll go all the way!
No, they won't make it out of Pool 5
No, they won't get past the quarter-finals
No, they won't get past the semi-finals
They'll lose in the final