Brash was taken out of Wednesday's game against the Twins due to side discomfort, Ryan Divish of The Seattle Times reports.
Brash entered for the eighth inning Wednesday and threw just two pitches before departing with the training staff. The severity of the injury isn't fully clear, but the right-hander will have a team day off Thursday to aid in his recovery before the Mariners begin their next series against the Royals on Friday.