Scarlets back Rhys Patchell is a doubt for Wales' Six Nations opener against France after suffering a hamstring injury in the Pro14 loss at Ospreys.
The 25-year-old will be sidelined for four to six weeks, making him a doubt for the trip to Paris on 1 February.
Patchell, capped 11 times, could also miss the following game with Italy in Rome on 9 February.
Scarlets lock Jake Ball is also a Six Nations doubt after suffering a shoulder injury against Ospreys.
Ball will be out for between three and six weeks but Scarlets coach Wayne Pivac - who will take over as Wales coach after the 2019 World Cup - says the 27-year-old does not require surgery.
Patchell missed almost two months with a head injury suffered against Ospreys on 6 October before featuring for Wales against Tonga on 17 November.
Pivac moved Patchell from fly-half to full-back against Ospreys with Wales centre Hadleigh Parkes filling in at 10.
The Scarlets coach says Patchell's defence has been affected by the concussion problem.
"He will be the first (defence) to admit it is a work-on in his game," said Pivac.
"Since he has had the last head injury it has been exposed a bit more.
"Rhys is out of the game for a bit now after picking up a hamstring injury which we have had scanned.
"So he will have time to get himself mentally and physically in the best shape possible.
"He will take a break from the game now and hopefully come back in much better shape."
Wales full-back Leigh Halfpenny trained on Thursday and could feature on Saturday against Cardiff Blues as he continues his recovery from concussion.
Halfpenny has not played since 10 November against Australia, when the Lions star was hurt by a challenge from Wallabies centre Samu Kerevi.
"Leigh trained today and we will see how he goes between today's session and the Captains Run for tomorrow (Friday)," said Pivac.
"So he has a chance but we will not risk him if he is not cleared for the weekend."