Ulster centre Jared Payne is set to miss Ulster's opening two European Champions Cup fixtures after undergoing surgery on a broken bone in his foot.
The Ireland squad member sustained the injury in the World Cup Pool D win over Italy and is expected to be out of action for at least six to eight weeks.
It had initially been thought that the 29-year-old had suffered bruising but a subsequent scan revealed a fracture.
Payne will also be ruled out of a number of Ulster's Pro12 matches.
Ulster face Cardiff Blues, Munster and the Dragons in consecutive weeks, before beginning their bid for European glory against French outfit Oyonnax on 14 November, followed by a home game against Saracens six days later.
He was ruled out of the remainder of Ireland's World Cup campaign ahead of the pool decider with France in Cardiff on 10 October and his absence over the coming weeks will be a major blow to the Irish province.
With fellow centre Luke Marshall also ruled out until the end of November and Stuart Olding a long-term injury absentee, Ulster's options in midfield are somewhat limited for a busy period of upcoming fixtures.
Winger Tommy Bowe is another injury concern for Ulster and scan results to ascertain the extent of a knee injury suffered in the World Cup quarter-final defeat by Argentina are expected on Tuesday or Wednesday.
Meanwhile Ulster are in discussions with the Irish Rugby Football Union (IRFU) about the availability of their international players, especially those who did not have much game time during the World Cup.
Coach Neil Doak has declined to speculate on when his Ireland players may return to action, but he does expect some of his Irish contingent to be available for the trip to Munster on 30 October, at the latest.
Ulster lie fifth in the Pro12 table ahead of Friday night's game with the Blues at Kingspan Stadium, following home victories over Ospreys and Treviso, but losses on the road against the Scarlets and Edinburgh.