The Giants selected Harrison's contract from Triple-A Sacramento ahead of his major-league debut Tuesday at Philadelphia.
Harrison has officially arrived in the majors just 10 days after his 22nd birthday and will try to help plug the leak in San Francisco's injury-ravaged rotation. The young left-hander was showing an underwhelming 4.66 ERA through 65.2 innings of work this summer at the Triple-A level and will likely be on a limited pitch count after missing most of July due to a Grade 2 hamstring strain, but he possesses an elite strikeout arsenal and should offer considerable fantasy appeal down the stretch.