ULSTER were dealt a huge blow on Sunday when it was confirmed captain Johann Muller was likely to be out for up to eight weeks.
The South African lock - who made his 50th appearance for the Province in the 19-8 win over Glasgow on Friday night in Scotstoun - suffered a hand injury during the match.
A scan later revealed he had ruptured ligaments around his thumb and he underwent surgery on Sundayto repair the damage.
However, it does mean the instrumental player could miss the crucial Heineken Cup Pool Four games against Northampton in December.
The valuable away win over Glasgow on Friday night left Ulster top of the pool and extended their unbeaten record in all competitions to seven - the only side in Europe not to have lost a game this season.
The Province are three points ahead of second placed Northampton Saints, who picked up a losing bonus point against Castres on Friday night.
It means the back-to-back games with the current English Premiership leaders will be crucial to the final outcome of the Pool and Muller’s absence is a huge blow if he was unable, as expected, to play.
He has missed just one game this season away to Cardiff Blues.
Meanwhile, Irish international backrow, Stephen Ferris’ back injury will be further assessed on Monday.
It is likely he will not be involved in Friday’s PRO12 match away to NG Dragons - and there is also the threat now that he could miss the Ireland Guinness Test series in November.
Ireland are due to name their squad on Wednesday, with Ferris expected to be included, unless Monday’s news proves otherwise.