Rhys Ruddock has flown home to Ireland for further treatment in a bid to save his World Cup hopes.
It remains unclear whether the 24-year-old has suffered a second broken arm in six months.
The Leinster back-row was injured during Emerging Ireland's victory over Uruguay in the Tbilisi Cup in Georgia on Wednesday.
However, X-rays at a Tbilisi hospital proved "inconclusive" due to the limited facilities.
Emerging Ireland team manager Joey Miles said: "Rhys is out of the tour and has flown home for further medical assessment.
"The X-ray was inconclusive but we wanted to err on the side of caution because he has suffered a previous injury to the same arm.
"Clearly he was disappointed to leave and to suffer the injury. He was very happy leading the team in Georgia and with his performances so far. But he knows it is for the best if he returns to Ireland for the sake of his immediate career."
Ruddock, who has won five caps including the win over South Africa last November, missed the Six Nations after he broke his arm against Harlequins in December. He then underwent surgery in April following an unsuccessful return.
He was handed the captaincy of Emerging Ireland in a bid to play his way back into contention for Joe Schmidt's World Cup squad.
Elsewhere, European champions Toulon have completed the signing of Stormers and South Africa back-row forward Duane Vermeulen, who was voted his country's best player last year.
The 28-year-old Springbok No 8 has signed a three-year contract which will keep him at the French club until 2018.
Vermeulen, who will sit out the Super Rugby play-off against ACT Brumbies tomorrow with a neck injury, will join Toulon after the Rugby World Cup in September.
Vermeulen's signing followed one day after Montpellier confirmed the arrival of Springbok brothers Bismarck and Jannie du Plessis, members of South Africa's 2007 World Cup-winning team, from the Sharks on three-year deals.
Around 30,000 extra tickets for the Rugby World Cup are to go on sale next week, including some for Ireland's match against Romania at Wembley.
Rugby fans have already snapped up more than two million tickets, and tournament organisers have now increased availability from returns via commercial programmes, extra seats at the venues and re-sales from those who can no longer attend.
Three thousand will be for the opening ceremony and England against Fiji on September 18, with another 4,000 for each of the quarter-finals held at Twickenham and England's match against Uruguay in Manchester.
Extra tickets are also being made available for Ireland v Romania at Wembley, France v Italy and Australia v Wales, as well as New Zealand v Tonga at St James' Park in Newcastle.
Managing director of England Rugby 2015 Steve Brown said: "With over two million tickets sold and less than 100 days to go, the appetite to watch the world's best teams is continuing to grow.
"This latest release of tickets will see further availability for five big matches at Twickenham, including England's opening game against Fiji and the two quarter-finals, as well as England versus Uruguay, giving fans who missed out last time another opportunity to come to the tournament."