Scarlets suffer huge setback
Halfpenny, who was instrumental throughout Scarlets' season, failed to recover from a hamstring injury he sustained during Scarlets' impressive win over the Cheetahs two weeks ago.
"Leigh is out," Scarlets head coach Wayne Pivac confirmed the news to WalesOnline, adding: "There was a small tear there, but with any luck should we get through the semi-final he may be available for the final,"
The coach is forced now to depend on New Zealander Johnny McNicholl, who has been cleared to return to the field after aggravating a shoulder injury against Leinster at the RDS in February.
"Johnny has been training really well for the last couple of weeks. The last obstacle for him was full contact and he has done that and come through so he is available for selection," Pivac said.