Wales forward Taulupe Faletau could miss the whole of the Six Nations after English Premiership side Bath said on Tuesday he had suffered medial knee ligament damage.
The British and Irish Lions player was hurt during his side's 24-20 European Champions Cup defeat in Toulon on Saturday and is likely to be sidelined for between 12 and 16 weeks.
Faletau's fellow 2017 British and Irish Lions Jonathan Davies and Sam Warburton are already sidelined from Wales coach Warren Gatland's plans as long-term injury absentees, while lock Jake Ball is currently battling to overcome a dislocated shoulder
Wales kick off their Six Nations campaign against in-form Scotland on February 3, followed by a Twickenham trip to face title-holders England seven days later, then Ireland in Dublin on February 24.
A 12-week absence would rule Faletau out of Wales's first three Six Nations games, but the whole tournament could prove off-limits for him, with Wales also facing Italy on March 11 and France six days later.
"We're really disappointed for Taulupe," said Bath director of rugby Todd Blackadder.
"He's been in great form this season so it's really unfortunate news.
However, he's in great hands with the medical team and we look forward to having him back on the pitch soon."
Faletau will also miss Bath's final three European Champions Cup pool matches, starting with Toulon's visit on Saturday, followed by two more encounters with Scarlets and Treviso in January.
The 27-year-old has won 70 caps for Wales and featured on the past two Lions Tours.