Jared Payne has been ruled out of the remainder of the Lions tour amid an uncertain recovery period owing to migraine symptoms.
The luckless Ireland centre had been dogged by a calf problem for the majority of the Lions' New Zealand tour - but is now suffering from migraines.
The 31-year-old was a late withdrawal before kick-off from the Lions' 31-31 draw with the Hurricanes in Wellington last week.
The Ulster star has stayed on in Wellington for further tests, and will continue to be stood down while his migraine symptoms persist.
Lions head coach Warren Gatland said: "While the findings of Jared's tests have been normal he still has migraines and will not be fit to train until his symptoms have settled.
"The specialists are happy that the symptoms are not concussion related and he is expected to recover fully. We wish him a speedy recovery."
The Lions will face New Zealand in Auckland in Saturday's Test series decider, but Ireland's Payne would have been unlikely to feature.
Payne suffered a lacerated kidney in Ireland's 27-24 win over Australia in November last year, and went on to battle calf issues in the latter half of the campaign.
The New Zealand-born centre fared remarkably in finding fitness at all for the Lions' tour of his homeland.
But now he faces another period on the sidelines, with his recovery timeline still difficult to plot.
Lions medical chief Dr Eanna Falvey had explained on Tuesday that Payne had remained in Wellington to undergo further tests with specialists.
"He had a bang in the Chiefs game, but he was fully cleared from concussion," said Falvey.
"But he's been struggling with migraine and that's an independent process since then."
Asked how long Payne could be sidelined, Falvey continued: "Jared's slightly more difficult.
"Thankfully all his tests have been good. He's suffering from migraine symptoms at the moment.
"The specialist is quite happy with him. He'll have a couple more minor tests, and that will give us more idea. But while he still has a headache he won't be training."
Meanwhile, Liam Williams has been backed to shake off a leg problem in time for Saturday.
Williams sat out training in Queenstown on Wednesday, with "tightness" in his leg - but assistant coach Neil Jenkins insisted the Wales full-back will be fit to contest selection.
The Lions will chase only their second-ever Test series win in New Zealand on Saturday, but must end the All Blacks' 39-match unbeaten run at Eden Park.
Williams should start again at full-back provided he shakes off his minor complaint, but the Lions have Leigh Halfpenny and Jack Nowell both waiting to slot into the starting XV if required.
"He's just a little bit tight, but he'll be fine," said Jenkins. "It's probably precautionary, just a little bit of tightness."
Both the Lions and New Zealand will name their starting line-ups on Thursday.
The Lions will be expected to make minimal - if any - changes to the side that edged out the All Blacks 24-21 in Wellington in Saturday's second Test.
Should Williams fail to recover in time, Wales star Halfpenny would be a straight swap. Head coach Warren Gatland also could opt to shift Anthony Watson to full-back and start Jack Nowell.
The Lions meanwhile are understood to expect the All Blacks to promote Jordie Barrett into their starting line-up, to wear 15 for his full debut, aged just 20.
Beauden Barrett missed three eminently-attainable penalties as the All Blacks lost out in the second Test, and his younger brother could take the goal-kicking reins if he starts the decider in Auckland.
The All Blacks must also rearrange their midfield after Sonny Bill Williams' four-week ban following his red card in the second Test.
Anton Lienert-Brown could shift inside to 12 with Malakai Fekitoa starting at outside centre, or boss Steve Hansen could prefer to promote Ngani Laumape into the starting XV and the inside centre berth.
