Jackson was pulled from his appearance Saturday due to lower-back tightness, Maria I. Guardado of MLB.com reports.
The severity of Jackson's injury remains unknown, but now we know for sure that it's not related to the elbow he had Tommy John surgery on last season. The 31-year-old reliever holds a 2.16 ERA and 0.84 WHIP through 8.1 innings since returning from the injured list May 31.