Holmberg, a 6-foot-4, 220-pound left-handed pitcher out of Port Charlotte (Fla.) High School, was assigned to the advanced rookie Bristol team.
The 17-year-old Holmberg was taken by the White Sox with the 71st overall pick in the 2009 First-Year Player Draft and is one of 30 draftees to have signed with the team.
Holmberg went 7-0 with a 0.22 ERA during his senior season of high school, allowing just two earned runs during the year. He struck out 132 batters while walking 23. Opponents batted .118 off of Holmberg, who did not allow any home runs this season.
Holmberg had signed to play baseball for the Florida Gators.
The White Sox have yet to sign their first-round pick, outfielder Jared Mitchell. Mitchell is participating in the College World Series for Louisiana State and cannot sign until his season is complete.
Jesse Temple is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.