Pienaar suffered a knee injury in the defeat by Leinster and may also miss Ulster's two upcoming European games.
Front-rowers Rob Herring and Rodney Ah You will be out for four to six weeks after suffering knee ligament damage in the Leinster game.
Forwards Wiehahn Herbst and Roger Wilson have also suffered injuries.
Prop Herbst will be out for at least a month after sustaining a calf injury in training last week, while Wilson will miss the Scarlets match after straining a knee in the game at the RDS.
Media playback is not supported on this device
Wing Craig Gilroy remains on the absentee list after suffering an ankle injury in training and prop Kyle McCall's absence for four to five months has been confirmed by Ulster following surgery on his torn hamstring before Christmas.
Marcell Coetzee (knee), Peter Nelson (foot), Ricky Lutton (toe), Alan O'Connor (concussion), Stuart Olding (calf), Jared Payne (kidney) and Matthew Rea (shoulder) remains out of contention for Friday's game, although Robbie Diack and Ross Kane will be available following injury absences.
Ulster will also hope to have Ireland stars Rory Best, Iain Henderson and Paddy Jackson available after they were ruled out of the Dublin game because of the IRFU's player management programme.
South African star Pienaar is being forced to leave Ulster at the end of this season following a controversial decision by the IRFU and will be disappointed to miss the upcoming games as his departure looms.
After leading the Pro12 table following five opening wins, Ulster have slipped to sixth spot having lost four of their past six games.
Ulster do have a game in hand on the top five sides following the postponement of their home match against Zebre but defeat in Wales would leave Les Kiss's side at least eight points off a play-off spot.
Struggling Ulster have suffered further injury blows including the loss of South African back Ruan Pienaar for Friday's Pro12 game against Scarlets.