Johann Muller - who has played in all 18 of Ulster Rugby's Heineken Cup matches since he joined the club in 2010 - is in danger of being ruled out of the key Pool 4 back-to-back matches against Northampton Saints in December.
The Ulster captain and former Springbok lock has had surgery on a hand injury suffered in Friday night's 19-8 Heineken Cup win over Glasgow Warriors at Scotstoun.
A medical scan after the game revealed that he had ruptured ligaments around his thumb and on Sunday he was operated on to repair the damage.
Ulster top the group three points clear of Northampton Saints and with Muller likely to be out for eight weeks that could see him miss the Friday, December 7 trip to Franklin's Gardens to face the 2000 European champions and the quick-fire return at Ravenhill eight days later.
Muller captained Ulster to last season's final at Twickenham and has so far led them to Pool 4 victories over Castres Olympique and the Warriors.
Capped 26 times by South Africa, Muller has played 50 times for Ulster and has been club captain since August 2011. His leadership has been instrumental in helping Ulster to an unbeaten start in both the RaboDirect PRO12 and the Heineken Cup this season.
His current contract with the Province runs until the summer of 2013.
