Taulupe Faletau may miss the whole of Wales' Six Nations campaign after being ruled out for 12-16 weeks with a knee injury suffered playing for Bath.
The back-row forward suffered a medial ligament problem against Toulon on Saturday and will be sidelined until early March at least.
Wales begin the Six Nations against Scotland on February 3, while their final game is against France on March 17.
"We're really disappointed for Taulupe - he's been in great form this season, so it's really unfortunate news," Bath director of rugby Todd Blackadder said.
"However, he's in great hands with the medical team and we look forward to having him back on the pitch soon.
"These are opportunities where players can step up, and I've been thoroughly impressed by the way the likes of Paul Grant and Zach Mercer have performed over the last few weeks, and I'm sure they'll continue to do so."
British and Irish Lion Faletau missed three months of last season after suffering a knee injury on his debut for Bath.
Sam Warburton has already been ruled out of Wales' Six Nations campaign after knee surgery that will keep him out for six months.