Iain Henderson is in a race to make Ireland's Six Nations finish at the end of October.
But the Ulster skipper will miss his province's PRO14 run-in and the Champions Cup quarter-final clash with Toulouse after recently undergoing hip surgery.
The loss of Henderson is a big blow to Dan McFarland's squad as they look to hit the ground running in the PRO14 - they resume action against Connacht on Sunday week. They are due to face Toulouse in European battle on September 20.
Ulster say that Henderson will be out for a period of eight to 10 weeks, with an estimated return date of mid to late October.
Ireland are poised to resume their Six Nations challenge at home to Italy on October 24 before the final round clash in Paris against France seven days later.
Confirmation from Leinster last week that Henderson's second row Test colleague James Ryan will return to action in mid-September following shoulder surgery means that Andy Farrell won't be as concerned.
Henderson will push to be ready for a quick Test return, but it might be too much to ask a player to return to the international arena having not played since March.
The 28-year-old missed Ireland's last Six Nations game played before lockdown, the loss to England at Twickenham, due to the birth of his son.
Another Ireland international, Will Addison, is currently rehabbing a back injury and unlikely to be involved against Connacht, while Andrew Warwick is due to undergo hip surgery next week.
Better news for McFarland is that Greg Jones (ankle), Matthew Rea (ankle), Sam Carter (shoulder) and Matt Faddes (shoulder) will be ready for the restart having successfully recovered after surgeries.
