Jared Payne faces an uncertain recovery period after being stood down due to suffering from migraines.
The Ireland centre was withdrawn from the British and Irish Lions' clash against the Hurricanes last Tuesday after complaining of headaches, and now faces further tests as the problem continues to linger.
Lions medical chief Dr Eanna Falvey admitted there is no clear return date for Payne yet, despite the 31-year-old progressing through a raft of tests.
"He's actually suffering from migraine," Dr Falvey said of Payne, who has remained in Wellington after the Lions' second Test victory over New Zealand for further specialist testing.
"He had a bang in the Chiefs game (on June 20), but he was fully cleared from concussion.
"But he's been struggling with migraines and that's an independent process since then."
Payne missed the Lions' 31-31 draw against Hurricanes draw, and was then not involved in Saturday's 24-21 triumph over the All Blacks.
The Ulster flyer has stayed in Wellington while the Lions squad have moved on to Queenstown.
The tourists will face New Zealand in the third and decisive Test at Eden Park on Saturday.
"Jared's slightly more difficult," Dr Falvey said of Payne's recovery situation. "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."
Ireland and Leinster centre Robbie Henshaw could now face a four-month lay-off after tearing his pectoral muscle off the bone.
"The muscle needs to be reattached, and that's usually about a 16-week recovery," Dr Falvey said.
"He may do slightly better, but that's probably what we're looking at."
Dr Falvey defended the Lions' concussion protocol work during their New Zealand tour, clearing up any confusion over head knocks suffered by Joe Marler and Alun Wyn Jones.
England prop Marler and lock Courtney Lawes clashed heads in the Lions' 34-6 win over the Chiefs in Hamilton last month.
Lawes underwent and passed a pitchside head injury assessment (HIA), and returned to action. Marler played on, but Dr Falvey revealed the Lions spotted in footage the next day something they felt worthy of further assessment.
"All the HIA management is a collaborative event now. I'm not making a decision on my own about that any more," Dr Falvey said. "We've raised the stakes on how we do this considerably.
"One of our medical team sits in the box as a spotter. He has the broadcast feed and he can rewind that.
"I also have access to a system where I have 12 views to look at any impact and decide whether it meets the criteria for permanent removal.
"Added to that, there are three independent doctors reviewing that and watching the game themselves.
"We made a conscious decision that the independent doctor would do all the HIAs. We then remove any implication at all that we'd be favouring a player to go back on.
"The players always want to go back on. I've had a couple of very tough conversations in this tour when players have wanted to go back.
"Players have to look after themselves. But even with all that, things get missed.
"Joe Marler, for example, in the Chiefs match. Joe at the time was okay, and the doctors were quite happy with him staying on. The next day when we reviewed our own video, we saw that Joe had actually gotten up and fallen to the ground. So that fulfils the criteria for removal. So then we moved Joe through a gradual return to play.
"We use the best tools, but some things slip through the net.
"Between the match and being reviewed, he wasn't in any contact situation."
Lock Jones took a bang in the Lions' 30-15 first Test defeat to the All Blacks, but Dr Falvey confirmed he was reviewed and cleared to keep playing.
"He did get reviewed at the next break of play," Dr Falvey said of Jones' head injury scare.
"When you run onto the field there's a lot of moving parts out there, it's actually a reasonably dangerous place to be. If someone is unable to get up, we'll protect them while play goes on around them.
"But if someone is on the ground, if they get up and run off, it's a difficult thing to stop one of these guys from moving when they want to move."