Ulster scrumhalf Ruan Pienaar will miss Friday's Celtic Pro12 match against the Llanelli Scarlets after sustaining a right knee injury.

Pienaar, who suffered the injury during the defeat to Leinster on 31 December, is set to see a specialist.

Meanwhile, Ulster's South African prop Wiehahn Herbst has been ruled out for at least a month after suffering a calf injury in training last week.

Bok flank Marcell Coetzee (knee) is also unavailable for Friday's match.

Ulster have also lost front-rowers Rob Herring and Rodney Ah You, who will be out for four to six weeks after suffering knee ligament damage in the Leinster game.

After leading the Pro12 table following five opening wins, Ulster have slipped to sixth spot having lost four of their last six games.