The 30-year-old walked to the back of the mound after his 55th pitch, and he left after taking a few warmup tosses.
The injury could be to Liriano's oblique, according to Dejan Kovacevic of the Pittsburgh Tribune-Review.
Liriano is 1-6 with a 4.60 ERA in 14 starts this season after he went 16-8 with a 3.02 ERA last year.
The Pirates put right-hander Gerrit Cole (6-3, 3.64 ERA) on the 15-day disabled list on Sunday with right shoulder fatigue.
Liriano was removed from tonight's game with discomfort in his left side. — Pittsburgh Pirates (@Pirates) June 11, 2014