Taulupe Faletau has suffered a broken arm for the second time this season – dealing Wales a major Six Nations injury blow.
The Wales number eight returned to action for his club Bath against Heineken Champions Cup opponents Wasps on Saturday.
That was the 28-year-old's first appearance following a 13-week absence after he broke his arm earlier this term, missing Wales' four autumn Tests as a result.
And Bath have now confirmed another fracture, although no timescale has been given for how long he will be sidelined.
Press Association Sport understands that the latest fracture is again to Faletau's right arm.
"Bath Rugby can confirm that Taulupe Faletau suffered a fracture to his forearm in Saturday's Heineken Champions Cup fixture against Wasps," Bath said, in a statement.
"It is not yet known at this time how long Taulupe will be out of action."
Faletau, meanwhile, tweeted: "Rejoice in hope, be patient in suffering, persevere in prayer. In & out. I'll be back."
Wales kick off their Six Nations campaign against France in Paris on February 1, with head coach Warren Gatland due to name his squad on Tuesday.
Faletau, one of Wales' most influential players during an outstanding international career that has also seen him play Test rugby for the British and Irish Lions, faces an uphill battle to feature in this season's tournament.
In terms of the back-row, Gatland is without Cardiff Blues flanker Ellis Jenkins and Scarlets blindside Aaron Shingler due to knee injuries.
Ospreys forward Dan Lydiate, meanwhile, has been recovering from a biceps injury, and Dragons number eight Ross Moriarty has not played for a month because of concussion.
It is understood, though, that Moriarty is on course to make a Dragons comeback ahead of the Six Nations starting.
