Wales could be without star number eight Taulupe Faletau for the whole of this season's NatWest 6 Nations.
Faletau's fellow 2017 British and Irish Lions Jonathan Davies and Sam Warburton are already sidelined from Wales' plans as long-term injury absentees.
And Faletau might now join them after his club Bath said he faces a lay-off of 12-16 weeks with knee trouble.
With lock Jake Ball currently battling to overcome a dislocated shoulder, it means that Wales head coach Warren Gatland has suffered yet another blow almost two months out from the tournament.
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' first three Six Nations games, but the whole tournament could prove off limits for him with Wales also facing Italy on March 11 and then France six days later.
Bath have confirmed that 27-year-old Faletau suffered a grade three medial knee ligament injury.
The powerful number eight, who has won 70 caps and featured on the last two Lions tours to Australia and New Zealand, was hurt during Bath's European Champions Cup defeat against Toulon last Saturday.
It is understood that Faletau will see a specialist later this week to discover if surgery is required.
"We are really disappointed for Taulupe," Bath rugby director Todd Blackadder said in a statement released by the club.
"He's been in great form this season, so it's really unfortunate news.
"However, he is in great hands with the medical team, and we look forward to having him back on the pitch soon."
Blackadder told reporters on Tuesday: "It's a serious one. We are looking at anywhere between 12 and 16 weeks.
"It's obviously a bit of a blow. I spoke to him after four big (autumn) games for Wales about how he was feeling both physically and mentally, and he was really good.
"It was just one of those things. A guy, sort of off the ball, dives sideways through his knee. These things happen."
Faletau suffered two knee injuries last term, and his latest problem surfaced early in the second half of a 24-20 pool loss in Toulon.
A long-term absence would also impact severely on Bath, who remain firmly in the Champions Cup quarter-final hunt and have a number of Aviva Premiership games on the horizon, including a home clash against Wasps.
And for Gatland, Faletau's unavailability means he has a considerable void to fill in the back-row.
Gloucester forward Ross Moriarty, who would be a stand-out candidate to replace Faletau, has only played one game this season due to injury, and Gatland could decide to switch the versatile Josh Navidi, who started at openside flanker with impressive effect in three of Wales' recent autumn Tests.
Source: PA
