To make room for Guilmet on the 40-man roster, the Orioles have transferred outfielder Nolan Reimold to the 60-day disabled list.
Guilmet, 26, owns a lifetime 2.57 ERA and 90 saves in 196 games (15 starts) over five minor league seasons in the Cleveland system. He's struck out 9.8 batters and walked 1.9 per nine innings.
Last year, Guilmet posted a 1.68 ERA over 64 1/3 innings, recorded 20 saves and struck out 10.1 batters per nine innings in 49 games with Triple-A Columbus.
The Indians selected Guilmet in the ninth round of the 2009 First-Year Player Draft out of the University of Arizona. His only major league experience came last season, when he appeared in four games and allowed six runs and eight hits, with three walks, in 5 1/3 innings.
"He has command of his fastball, slider and split-finger, which he mixes to get hitters out," executive vice president Dan Duquette said in a text message.
"We expect he'll contribute to the major league team this season, given his track record and experience closing games."
Boss, an eighth-round pick in 2012 out of Michigan State, batted .238/.333/.358 last season in 106 games at Single-A Delmarva.
Reimold has been receiving treatment on his neck following corrective surgery over the summer to fuse two vertebrae. Manager Buck Showalter told reporters today that Reimold hasn't been participating in baseball activities.
Reimold was able to hit during exhibition games, but had difficulty playing the outfield. He's been shut down since the Orioles broke camp.