Liam Williams is set to miss Wales' Six Nations opener against Scotland and could sit out the entire tournament if he requires surgery
The versatile back made his Saracens return against Ospreys following a two-month absence due to an abdominal injury.
Wales head coach Warren Gatland revealed that the British and Irish Lion is still struggling just 10 days before Scotland visit the Principality Stadium on the opening day of the tournament.
"He is being rehabbed extensively over the next couple of weeks," said Gatland.
"Hopefully they can get him right but if he's not right unfortunately he might require surgery.
"It's probably an injury that six weeks ago he should have had an operation on it, but Saracens had rehabbed him and he's come away from the game against the Ospreys feeling very sore.
"Our medical team had a look and want to rehab him extensively over next couple of weeks."
News of Williams' setback comes after it was revealed Dan Biggar is expected to miss the first three rounds of the Six Nations.
Asked how he plans to replace the fly-half, Gatland said on Wednesday: "We've got three options. From an attacking perspective, it's pretty exciting in terms of which way we go.
"Owen Williams has a bit of a disadvantage because he's had to go back to Gloucester. The English-based players have that disadvantage. He'll have to go back in the fallow weeks as well. The exciting thing is that we're going to pick a player who hasn't started in that position in the Six Nations.
"Opposition teams know Dan Biggar pretty well, so we could potentially have a fly-half that the opposition don't know a lot about."