Racing Metro have confirmed that Jonathan Sexton is out of action until mid-February, thus missing the start of the Six Nations.
The out-half suffered a concussion following a clash of heads with Rob Kearney during Ireland's victory over Australia, and has not played since.
A statement on Racing's site says the concussion is Sexton's fourth in 12 months, and that the club must follow World Rugby and French league protocol in keeping Sexton on the sidelines for 12 weeks.
Sexton faces a consultation on his condition on February 7th, which puts him out of Ireland's opening Six Nations game with Italy.
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