Wales full-back Halfpenny, 30, has already missed two months with concussion after suffering a head blow against Australia on 10 November.

"Leigh has seen the specialist and won't be used for the next three to five weeks," said Pivac.

Wales' Six Nations campaign starts against France in Paris on 1 February.

Pivac says Halfpenny will be assessed in around a month.

"He'll continue training and doing what he's been doing, and then he will be reassessed," said Pivac.

"It will be somewhere between three to five weeks, but these things take time.

"We will pass on the medical information to the Welsh Rugby Union. That will then be Warren's [Wales coach Warren Gatland] call as to what he does because the tournament runs for longer than that.

"He's running, he's training, and from time to time I think he's been having bouts of [headaches].

"He's as frustrated as anybody is, but his health and wellbeing comes first.

"Lots of players have come back from concussion before. Justin Tipuric and Aaron Shingler have had spells out and both those players have returned to the top of their game at some point."

Pivac also said he hopes second-row Jake Ball (shoulder) and fly-half Rhys Patchell (hamstring) will be fit for the start of the Six Nations and the pair could feature in the Scarlets' Pro14 match against Leinster on 25 January.

British and Irish Lions centre Jonathan Davies will be rested for the Champions Cup game against Leicester on 12 January. Pivac confirmed Wales wing Steff Evans would feature for Scarlets in the final two rounds of Europe.

Gatland also has concerns over James Davies (knee), Nicky Smith (ankle), Dan Lydiate (elbow), Ross Moriarty (concussion) and Bradley Davies (knee) for the start of the Six Nations.

Bath back-rower Taulupe Faletau has not played since breaking his arm in October, while Liam Williams only returned for Saracens in Friday's defeat against Sale after a six-week break with a hamstring problem.

Scarlets flanker Aaron Shingler will not return until March from a knee injury suffered in May 2018, while Cardiff Blues captain Ellis Jenkins is a long-term casualty.