The Pirates optioned Holmes to Triple-A Indianapolis on Monday.
Holmes was solid if unspectacular during his first season at Indianapolis last season, posting a 3.36 ERA, 1.38 WHIP and 99:59 K:BB in 112.2 innings in the International League. That performance wasn't enough to give him a realistic shot at cracking the Pirates' rotation out of spring training, however, so the 24-year-old will head back to the minor-league ranks and bide his time until injuries open up a spot for him with the big club.