Pienaar ruled out of Ulster clash
The right knee injury occurred during the match against Leinster.
The 32-year-old will see a specialist this week to decide on further management.
Also in the Leinster game, Rob Herring and Rodney Ah You sustained knee ligament injuries and the initial prognosis is that both players will be unavailable for selection for at least six weeks.
Roger Wilson also sustained a knee injury in the Leinster game and he too is unavailable for Friday's game against Scarlets.
Wiehahn Herbst sustained a calf muscle tear in training last week and it is estimated that he will be absent for four to six weeks.
Craig Gilroy suffered an ankle injury in training and will not participate in the Scarlets fixture.
Kyle McCall has had surgery to repair a torn right hamstring (sustained in the Clermont away fixture) and the recovery period for this injury is likely to be up to four months.
Also unavailable for selection this week are Marcell Coetzee (knee), Peter Nelson (foot), Ricky Lutton (toe), Alan O'Connor (concussion), Stuart Olding (calf), Jared Payne (kidney) and Matthew Rea (shoulder).
Ross Kane, Robbie Diack, John Donnan and Aaron Cairns have all recovered from recent injuries and are available for selection.
