Wales lock Jake Ball suffered another shoulder injury during the Scarlets' 19-12 derby defeat at the Ospreys.
The 27-year-old was substituted at half-time at Liberty Stadium and awaits a scan to determine the damage.
He then suffered another shoulder setback after his return this season.
Scarlets head coach Wayne Pivac said: "He felt a shoulder early in the match and got through to half-time.
"We'll get that scanned."
Pivac also has concerns after his full-back Rhys Patchell limped off during the game, but said "the main one is Jake Ball".