The injury to the North Hollywood (Calif.) Harvard-Westlake right-hander was first reported by Eric Sondheimer of the L.A. Times; the initial diagnosis is that it is a season-ending injury but not necessarily a career-threatening one.
His high school coach Matt LaCour told the paper he is likely to miss the next six to 10 weeks.
"It's not torn up to where he needs surgery," LaCour told the paper. "He's probably done for the season."
Giolio was projected to be as high as the No. 1 overall pick in the upcoming MLB Draft in June - a position where no other high school right-hander has ever been taken - but this will certainly hurt those projections regardless of the severity of the injury.
The 6-foot-6 pitcher is committed to UCLA and was recently in the spotlight for hitting 100 miles-per-hour with a pitch against Woodland Hills (Calif.) El Camino Real just over a week ago.
Giolito was pitching in the seventh inning against Mission Hills (Calf.) Alemany when he felt the discomfort and was removed from the game.
LaCour initially told reporters that Giolito had a groin injury but later revised his story.
"He's crushed," LaCour told the paper. "His coaches are crushed, and we're going to have to spend the next two days getting back on track."
Giolito is scheduled to have an MRI later Wednesday.
His draft stock will be re-evaluated with the diagnosis.