Cardiff tight-head Dillon Lewis will be out until potentially the end of January with a knee ligament injury.
The 26-year-old, who has been linked with a move to England amid the current pay freeze in Wales, last featured against Brive on December 10 and is now not expected to play again until the return fixture with the French outfit on 21 January.
Even that is an optimistic assessment, with Cardiff boss Dai Young admitting that the January 28 clash with Leinster seems a more likely return date.
Read more: Scarlets face fresh blow over Wales star's fitness just weeks out from Six Nations
As well as leaving Young without an experienced international for the remaining Welsh derbies and the start of the second block of European fixtures, it also casts doubt upon Lewis' Six Nations involvement.
Returning Wales coach Warren Gatland is set to name his squad around January 17, before Lewis is set to return, while the first match of the tournament against Ireland is on February 4. Lewis has started seven of the last eight Wales matches, but this setback gives him little time to recover before the tournament kicks off.
"Dillon is a good bit away," said Young. "I think he's in a brace for four or five weeks, so he won't play any of these derbies or the start of Europe. He might make the Leinster game or maybe Brive.
"The best picture would be Brive. Probably the more realistic one is Leinster."
However, Young has had some more positive news on the injury front ahead of the New Year's Day clash with the Ospreys, with Liam Williams available for selection for the first time since a collarbone injury on the opening day of the season. Josh Adams has also recovered from a bump in the win over the Dragons to be available.
That means Young has two Lions at his disposal in his back-three, while Young added that fellow back-three options Matthew Morgan and Jason Harries are also available.
"Josh Adams is fine," said Young. "There's been no issues with him. Liam Williams will also be available for selection.
"It's what they bring not only on the field with their quality that everyone can see, but their drive and attitude through training in the week.
"It helps get the preparation right. I'm a big believer that if you get the training right, it filters into the game. Obviously, they're two quality individuals who set high standards.
"Rehab is a lonely old place and that's the last thing when you come to a new club. Liam is professional enough to get on with it and is ready to show us all what he can do.
"Josh Adams is fine and Liam Williams is fine. You'd be a brave man not to pick them two."
However, there is doubt over Rhys Priestland and Theo Cabango after they departed the victory at Rodney Parade with knocks. Young admitted it was touch and go whether either would feature this Sunday.
"Rhys is still struggling at the minute, while Theo is another of our two major concerns for the weekend," he said. "They're not ruled out yet, but we'll probably take until Saturday for the team run.
"It's an ankle strain (with Rhys). Nothing structural. He's been icing it for 48 hours.
"It's touch and go if he'll make it this week, but we're not writing it off - just like Theo. But if he doesn't make this week, I'm sure he'll be alright next week.
"Theo, we think he's caused some ligament damage in his shoulder. He's had an MRI scan and he's off to see the specialist today. It's not as sore as was expected, but that's not always a good or bad sign."
Wales back-row James Botham is expected to be fit for the January 7 clash with the Scarlets at the Arms Park.
READ NEXT
The Welsh rugby headlines you can expect in 2023 with series of major announcements imminent
Eight nailed-on Wales starters for Six Nations opener with Ireland in 39 days
