An MRI conducted on the 25-year-old Monday showed a mild strain that's expected to keep him out of action for three-to-four weeks, Fegan adds.
"I thought it was a cramp in my triceps," Giolito said, according to MLB.com's Scott Merkin. "So, I was like, 'I don't need to throw a side. I'm good.' Then the MRI showed it was like a very, very mild lat strain. So they decided to shut me down."
Giolito made his first All-Star team in July. He's a candidate for the American League Cy Young award and Comeback Player of the Year after posting a 14-9 record with a 3.41 ERA and 1.06 WHIP across 29 starts.
"For me, it just sucks because I wanted to finish what I started," Giolito added. "But at the same time, it is what it is. It just was kind of a random thing that happened. That's it."
The White Sox ace pitched a career-high 176 2/3 innings in 2019 and is tied with Cleveland Indians right-hander Shane Bieber for the most complete games (three) in MLB. He also pitched two complete-game shutouts against the Minnesota Twins and Houston Astros earlier this season.
Giolito struggled in 2018, fashioning a 6.13 ERA while leading the AL in walks and allowing the most earned runs in baseball.