Wales prop Samson Lee has been ruled out of Sunday's RBS 6 Nations clash against Scotland at Murrayfield.
Lee suffered a knock to his head during last week's Millennium Stadium defeat against England.
The Welsh Rugby Union said on its official Twitter feed that Lee "was unable to complete the final component of the return to play protocol and has been ruled out of the second RSB 6 Nations fixture."
Lee has been replaced in the starting line-up by Ospreys tighthead Aaron Jarvis, with Cardiff Blues forward Scott Andrews named on the bench.
Lee's absence is a blow for Wales, and comes after it was decided earlier this week to rest wing George North for the Edinburgh trip.
North has been given an extended recovery period by Wales and he will not be involved in the Murrayfield encounter.
He suffered two heavy blows to his head during the England game, and although the Welsh Rugby Union has consistently said he has been symptom-free in terms of concussion, he will now spend time recovering.
North, 22, was concussed when Wales lost to world champions New Zealand last November, and he sat out the following weekend's Millennium Stadium appointment with South Africa.
Rugby union's world governing body World Rugby said earlier this week that North should not have remained on the field against England after a clash of heads with team-mate Richard Hibbard.
But World Rugby added that it accepted the WRU's explanation surrounding the incident.
North appeared to suffer a momentary loss of consciousness following the 61st-minute collision with Hibbard. WRU national medical manager Prav Mathema subsequently admitted that had the second incident been seen by medics then North would have been immediately taken off and replaced.
North left the field in the first half after receiving an accidental boot to the head from England lock Dave Attwood, but he was able to continue after treatment.
North had been replaced in the Wales starting line-up by Scarlets back Liam Williams, and Wales head coach Warren Gatland has now made a second switch as his players look to bounce back following the England loss.
Lee, meanwhile, had been undertaking graduated return to play protocols, and he was named in the starting XV on Wednesday, subject to those tests being successfully completed.
Speaking during a press conference at the Wales training base on Friday, Wales assistant coach Rob Howley said: "Obviously, Samson hasn't completed the final component of the concussion test.
"It's obviously disappointing to lose a player of his ability.
"Aaron and Scott have been with us over the last couple of years, and Aaron came to South Africa with us last summer and came on in the second Test.
"It's a great opportunity isn't it. We feel for Samson, as coaches do, but it's important we move on and look forward to the opportunity it presents to both Scott and Aaron."
Asked if Wales had considered contacting 95 times-capped prop Adam Jones, who recently announced his retirement from Test rugby, Howley added: "We respect players' decisions, and Adam has retired from international rugby.
"If Adam had made the call to Warren, I'm not too sure, but I'm unaware of that.
"Aaron has been here all week because Samson hasn't taken part in training, and Scott has been there as well."