The goal-kicking Wales and British and Irish Lions full-back has been sidelined by a torn hamstring.
Halfpenny was instrumental in the Scarlets’ run to this season’s European Champions Cup semi-finals.
Head coach Wayne Pivac, though, says there is a chance that Halfpenny could be available for next week’s PRO14 final in Dublin if Glasgow are toppled.
Full-back and wing option Johnny McNicholl is available for selection after overcoming a shoulder injury.
“Leigh has a small tear there, so with any luck, should we get through the semi-final, then he may be available for the final,” Pivac told BBC Sport.
“Johnny has been training really, really well for the last couple of weeks.
“The last obstacle for him was full contact, and he’s done that and come through, so he’s available for selection.”