Iain Henderson is in danger of sitting out Ulster's Heineken Champions Cup games next month because of a hamstring injury.
Henderson was due to start for Ireland against Argentina last Sunday, but was replaced at the last minute by Tadhg Beirne after suffering the injury.
Henderson will definitely miss this Saturday's United Rugby Championship clash with Leinster.
And he is likely to miss the European fixtures with Clermont and Northampton Saints next month.
"It'll certainly be a few weeks," said Ulster boss Dan McFarland, who is hoping to guide the team to a first win against Leinster in Dublin in eight and a half years.
More positively for the province, John Cooney is expected to return to this week after a series of injury issues, including neck and hamstring problems that have kept him sidelined for a long period
Ulster's Ireland contingent, except for their skipper Henderson, are ready to go against Leinster
McFarland also says all of his returning internationals (except Henderson) will be available for selection against the defending champions.
The head coach is unsure how he will deal with his Ireland players' game-time schedules as yet.
"They're all available to play, there is nobody who is not allowed to play," he remarked.
"They were obviously all part of the player-management program so it's a question of whether it best suits to play them this week.
"Some guys played the weekend, some guys need a break, it's all done on an individual basis, so we're going to be making decisions on a combination of what's best for us and what's best for individuals.
"I think we approach it for what it is, it's a block of 10 games where we play seven different teams. It's a hugely challenging block having Leinster twice, Munster, Connacht and four big European games as well as the Ospreys and the Scarlets to finish off is a huge challenge.
"It's probably the main course of our season, will go a long way to determining what the dessert looks like."
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