Pienaar, who is leaving Ulster at the end of this season, sustained the injury in the warm-up before Saturday's defeat by the Ospreys.
The South African, 33, started Saturday's contest but was forced off after 20 minutes.
Ulster's hopes of reaching the play-offs semi-finals now look remote.
Les Kiss' side are now five points behind fourth-placed Ospreys and only a huge bonus-point victory over the leaders this weekend - allied to a heavy Ospreys defeat - would see Ulster sneaking into the semi-finals.
Pienaar will round off his Ulster playing career in the game the Barbarians at Kingspan Stadium on 1 June.
Stuart Olding will miss the Leinster game after picking up a leg injury in Saturday's defeat while Charles Piutau also picked up a knock in the weekend game.
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Ruan Pienaar may have played his final Pro12 game for Ulster as a back spasm means he is a doubt for Saturday's contest with Leinster.