Ireland second row Dan Tuohy is set to be ruled out for up to eight weeks after suffering a thumb injury in Ulster's 24-20 Pro12 win over Treviso.
The lock, who has agreed a new three-year contract with the Irish province up to the summer of 2018, will undergo surgery to repair a torn ligament.
The 29-year-old will be unavailable for most of Ireland's Six Nations campaign.
Tuohy has already missed a lot of this season with an arm injury sustained against Cardiff Blues in September.
The former Exeter Chiefs player has been capped nine times by Ireland and would have been expected to be part of coach Joe Schmidt's squad for the Six Nations in February and March.
Meanwhile, prop Wiehahn Herbst will miss Ulster's European Rugby Champions Cup pool match against Toulon in France this weekend after sustaining a hamstring injury in Sunday's victory in Italy.
Full-back Ricky Andrew is also ruled out of the clash with the European champions because of a perforated eardrum.
Centre Luke Marshall is set to be absent as he follows the return-to-play protocols after suffering concussion in a training session last week.
"Our resources are very stretched for the trip to Toulon but we will pick our strongest side from the players that we have available," commented Ulster coach Neil Doak.
"The pressure is on Toulon but we'll go there with nothing to lose, try and get a win and use it as a springboard for the rest of the season."
On a more positive note, back row Nick Williams is likely to be available for selection before the end of January following a hamstring injury and lock Iain Henderson has taken part in some light training after being out since September with a hip injury.
Ireland wing Andrew Trimble's target date for a return after toe surgery remains mid-February, while centre Stuart McCloskey is expected back in six to eight weeks following elbow surgery.
Flanker Chris Henry (heart surgery) and Ronald Raymaakers (hamstring) remain sidelined but prop Ruaidhri Murphy has recovered from the finger injury he sustained playing club rugby for Ballynahinch in November.