Faletau, who has won 70 caps and featured on the last two British and Irish Lions tours, was hurt during Bath’s European Champions Cup defeat against Toulon last Saturday. And Bath have now confirmed that he suffered a grade three medial knee ligament injury, which is likely to see him sidelined for 12-16 weeks.
Faletau will see a specialist later this week to discover if surgery is required.
Wales kick off their Six Nations campaign against Scotland in Cardiff on 3 February, followed by a trip to face England at Twickenham seven days later.
“We are really disappointed for Taulupe,” Bath rugby director Todd Blackadder said in a statement. “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.”
A 12-week absence would rule him out of Wales’ first three Six Nations games against Scotland, England and Ireland. But the whole tournament could prove off limits for Faletau with Wales also facing Italy on 11 March 11 and then France six days later.
Wales are already without two of Faletau’s fellow Lions for this season’s Six Nations, with centre Jonathan Davies and flanker Sam Warburton both sidelined.Lock Jake Ball is recovering from a dislocated shoulder, meaning he has little chance of featuring during the early Six Nations action.
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.
For Wales head coach Warren 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 versatile openside Josh Navidi to No 8.