Conor Murray will have to prove his match fitness before the end of the season or risk missing out on the British & Irish Lions tour.
Scrum-half Murray, who was included in the 41-man squad for New Zealand, was named in the party despite having not played since Ireland's match against Wales on March 10 when he was forced off with a shoulder injury.
Warren Gatland, the Lions coach, described it as a "nerve issue" on Wednesday with Murray set to miss Saturday's Champions Cup semifinal for Munster against Saracens.
"He had a setback a couple of weeks ago," Gatland said. "He's going to need to play before he gets on the plane. That's going to be important for us.
"I would be reluctant to take someone to New Zealand with a neck injury who hasn't played particularly. His last game was against Wales, on March 10 I think.
"He's incredibly fit as a player. It's causing him a bit of discomfort and a bit of weakness in his arm, and hopefully he's going to be right in a couple of weeks, and the neurologist is confident he's going to be okay. He's had some disc problems in the past but that's not what's causing the issue.
"It's a concern, because he is a quality player. I think he's disappointed himself. He was hoping to be right this week for Munster. So hopefully he's going to be right in a couple of weeks and get one or two games under his belt."
Captain Sam Warburton is also sidelined, with a knee problem, as is Wales teammate Alun Wyn Jones who is struggling with a shoulder injury but Gatland is optimistic he will be fit before the end of the domestic season.
"He's making really good progress and again they're confident he's going to be right," Gatland said of Jones. "It's not like a hamstring or a muscle strain, where you know there's a time frame and players can recover from it.
"He's got a little bit of a shoulder issue. He's lifting some heavy weights in training but again you'd like to see him having a run under his belt for the Ospreys before the tour as well to make sure that we could be confident with some tough contact sessions, that we know that shoulder issue is going to be right.
"The prognosis with him is a lot more positive than was first diagnosed in terms of it's an unusual injury with a capsule thing, or something, but the prognosis and the recovery is hopefully a lot better than what was potentially first suspected."