Rhys Webb could have played his last game for Wales after he was ruled out of the entire Six Nations with a knee injury.
The Welsh Rugby Union said the scrum-half will undergo further assessments "to establish the best course of action" but the 29-year-old has been replaced by the uncapped Tomos Williams.
Webb's withdrawal could spell the end of his Wales career. He joins Toulon from the Ospreys at the end of the season and will be ineligible for selection as overseas players must have won 60 caps for their country.
Webb has represented Wales 31 times.
It is the latest in a host of injury setbacks for Warren Gatland. Sam Warburton, Jonathan Davies, Dan Lydiate and Jake Ball will miss the entire tournament while Taulupe Faletau and Dan Biggar are out of Wales' first three fixtures.
Rhys Priestland is sidelined with a hamstring problem, meaning Gatland is without his two leading fly-halves, and Liam Williams could miss all the whole Six Nations with an abdominal injury.
Wales host Scotland on February 3 in their first fixture before travelling to Twickenham to take on England on February 10.
"Scotland first up is a good challenge for us," said Gatland. "If you can win those first two games, you have a chance of winning the championship.
"It is always a good position for us to be in where people tend to write you off.
"Our players have always thrived on being underdogs. People put the boot into them and they respond. It is a good position for us to be in."