Iain Henderson will be in a race to be fit for Ireland's rearranged Guinness Six Nations fixtures against Italy and France after Ulster confirmed the Lions lock has undergone hip surgery.
The 28-year-old is expected to be sidelined "for a period of 8-10 weeks" which would put his return date between mid and late October and likely to miss the remainder of Ulster’s season.
Dan McFarland’s side return to action on 23 August against Connacht at the Aviva Stadium and will take on Toulouse in the Champions Cup quarter-final in five weeks’ time, with the final set to take place on 17 October.
Ireland conclude their Six Nations campaign with a home tie against Italy on 24 October, followed a week later by a trip to Paris against fellow title challengers France.
The 55-cap international will hope to be involved for the one-off Eight Nations tournament in November where Ireland will play Wales, England and Fiji.
The games will be run off in four weeks beginning 14 November with the winners of each pool playing in the final, scheduled for 5 December.
Ulster are preparing for a return to action later this month and have a number of other injury concerns.
Irish international Will Addison is rehabilitating a back injury, Angus Curtis is still recovering from a multi-ligament knee injury, while Andrew Warwick is due to undergo hip surgery next week.
Greg Jones (ankle), Matthew Rea (ankle), Sam Carter (shoulder) and Matt Faddes (shoulder) have all returned to training following respective surgeries and are expected to be fit for selection for the interpro against Connacht.