Wales captain Gareth Thomas will miss the rest of this season's RBS Six Nations Championship after suffering damage to an artery in his neck.
The Toulouse star faces up to 12 weeks out, having undergone an MRI scan after going to hospital on Sunday night suffering from a severe migraine.
The No 8 Michael Owen replaces Thomas as skipper against Ireland in Dublin next Sunday, with Llanelli Scarlets' Lee Byrne taking over at full-back.
Thomas could conceivably be sidelined for the rest of this term, also dealing Heineken Cup quarter-finalists Toulouse a major blow.
He had hoped to travel to Dublin with his team-mates, but rest has now been advised and he will remain at home.
"The damaged blood vessel now needs to be given time to heal before Gareth can resume training and eventually playing," said Wales team doctor, Professor John Williams.
"We are expecting him to make a full recovery, and it is envisaged that he will be able to resume training after ten to 12 weeks."
Thomas appeared on a BBC Wales television programme on Sunday night, when he was grilled for 30 minutes about Mike Ruddock's shock departure as Wales head coach. He complained of feeling unwell shortly afterwards.
One other enforced change from the side that beat Scotland 28-18 nine days ago sees leg injury victim Shane Williams replaced on the left wing by Scarlets back Dafydd James, who wins his 44th cap.
Gavin Henson, meanwhile, is on course to make his first Wales appearance for almost a year.
Henson has not played for Wales since the Grand Slam-clinching victory over Ireland in Cardiff last March.
He missed the autumn Tests after undergoing groin surgery, and then incurred a lengthy suspension.
He is among three changes on the bench, with Henson replacing Nicky Robinson, Ospreys forward Jonathan Thomas taking over from Scarlets lock Adam Jones and full-back Barry Davies taking Byrne's bench role.
Wales, despite a 47-13 drubbing by world champions England in their Six Nations opener earlier this month, can still retain their title. But they have not toppled Ireland at Lansdowne Road since 2000, and defeat on Sunday would effectively destroy their championship chances.
