McFarland was optioned following yesterday's 5-2 loss to the Astros.
Manager Buck Showalter indicated yesterday that the club wanted a right-hander to match up against the Tigers, who visit Camden Yards for a three-game series.
It appeared that the Orioles sought a reliever who could provide length in the bullpen after optioning their only true long man, but Guilmet doesn't fit that description. He's mostly been limited to one inning while recording four of the Tides' six saves this year.
Guilmet is 2-1 with a 5.65 ERA in 12 appearances, with nine earned runs (11 total) and 11 hits in 14 1/3 innings. He's walked four and struck out 17.
Guilmet most recently tossed a scoreless inning on Saturday. His worst outing came two nights earlier in Louisville, when he allowed six runs and three hits and walked three batters in one-third of an inning.
That's one way to inflate your ERA.
The Orioles acquired Guilmet, 26, from the Indians on April 7 for minor league outfielder Torsten Boss. His only major league experience came last season, when he allowed six runs and eight hits in 5 1/3 innings over four relief appearances.
A corresponding move from the Orioles isn't necessary because Guilmet is on the 40-man roster.
The Orioles could have recalled Steve Johnson, who's on the 40-man, rested and capable of providing length. Josh Stinson also fits the description of a long reliever, but he's not on the 40-man.
Today's move will give the Orioles four right-handers and three lefties in the bullpen.