Former Wales flanker Martyn Williams says it will be a big setback if Taulupe Faletau misses the start of the Six Nations.
Faletau has been sidelined for four to six weeks after sustaining a knee ligament injury paying for Bath on Christmas Eve. Wales strat their Six Nations campaign against Italy on February 5 and reigning champions England six days later.
"It's a worry, it's a big concern. In my opinion he's Wales' best player," Williams told BBC Wales.
"He's been Wales' best player for the last four or five years so it's a big, big loss if he does not come through in time for that first game against Italy."
Faletau missed two months of action at the start of the season with the same injury after joining Bath from the Dragons in 2016. He missed three Tests during the November internationals before making a big impact off the bench in the 27-13 win over South Africa on November 26.
"Taulupe is such a cog in that machine. You saw when he came on, only for the second-half against South Africa, the difference he makes," added Williams, who made 100 appearances for Wales.
"The timing of his passes, his footwork, his ball carrying ability, everything, the full package. He is a miss, but there is a lot of class there who can cover him and hopefully it will be for one game at the most too.
"The back row against Argentina where it was Moriarty at eight, Sam Warburton at six and Justin Tipuric at seven worked really, really well. I suppose that is one silver lining. As long as those three get to the Six Nations fit there's a combination that has been tried before."