Pienaar and Williams set to return for Ulster trip to Glasgow
However, Ireland flanker Stephen Ferris will not start after failing to recover from a back ailment which ruled him out of the bonus-try victory over Castres last week.
Paul Marshall, the unfortunate two-try man-of-the-match from the 41-17 win over the French side last weekend, finds himself on the bench alongside the in-form Michael McComish as coach Mark Anscombe makes two changes.