Ulster and Ireland pair Andrew Trimble and Luke Marshall face lengthy spells on the sidelines because of injury.
Trimble has suffered ligament damage to a toe and the wing is expected to be out for a "period of months".
Centre Marshall sustained a medial knee ligament injury while playing for the Ravens last weekend and he will be out of action for six weeks.
The duo will miss Ireland's November Tests against South Africa, Georgia and Australia at the Aviva Stadium.
The news is also a major blow for Ulster as they prepare to take on Toulon in the European Champions Cup on Saturday.
Trimble was forced off with the injury in the first half of the Pro12 victory over Glasgow Warriors on 11 October.
The 30-year-old missed Saturday's 25-18 defeat by Leicester in the European Champions Cup Pool 3 opener at Welford Road, and a scan has revealed the full extent of the injury.
Ulster, along with Ireland medical staff, will seek a second opinion on the injury and an update will be provided later this week.
Marshall picked up his knee injury during the Ravens' game against Yorkshire Carnegie on Saturday.