Ulster centre Stuart Olding will be sidelined for up to nine months after suffering a serious knee injury against Cardiff Blues on Friday night.
A scan has confirmed that Olding has an anterior cruciate ligament (ACL) injury, which will require surgery.
Olding, who sustained a similar injury in 2013, lasted just a few minutes into his comeback against Cardiff after 10 weeks out with an elbow problem.
It ends Olding's hopes of making Ireland's World Cup squad.
The unlucky 22-year-old missed most of the 2013/2014 season after rupturing his ACL.
That injury, sustained while playing for Ulster Ravens in November 2013, kept him out of the game for 10 months.
He made a full recovery and scored a try for Ireland in the autumn series win over Georgia last November.
However, his progress was halted by an elbow injury sustained in the European Champions Cup defeat away at Toulon in January, which ruled him out of Ireland's entire Six Nations campaign.
Ulster had hoped his return, alongside that of fit-again fly-half Paddy Jackson, could help propel the province towards the Pro12 play-offs and potentially win the final, scheduled to take place at their home ground on 30 May.
Meanwhile, Ulster fly-half Ruan Pienaar suffered a shoulder injury in the victory over the Blues at Kingspan Stadium.
The South African has soft tissue bruising, but he is expected to be available for the Pro12 match against Connacht on 11 April.