John Maine, a non-roster invitee who opened the season with the Marlins, was designated for assignment on Friday.
In four relief appearances, the 31-year-old posted a 12.27 ERA in 7 1/3 innings.
Tom Koehler was called up from Triple-A New Orleans to fill the long relief, emergency starter role.
"Today was actually going to be [Koehler's] day to pitch, so he's ready to go," Marlins manager Mike Redmond said before Friday's game against the Reds. "We'll pitch him out of the bullpen. If we need him as a long guy, he can go multiple innings. Maine ... was kind of filling that role, so he'll take his spot."
The Marlins are off to a rough 3-13 start, and like the team, Maine has had his struggles.
The veteran right-hander threw two innings in Thursday's 11-1 loss at Cincinnati. He was tagged for six runs on four hits in two innings, with five walks and three strikeouts.
Maine broke in with the Orioles in 2004, and he pitched for the Mets from 2006-10 before experiencing shoulder problems.
Koehler made three starts for New Orleans, and in 17 innings he had a 1.06 ERA with 14 strikeouts and 10 walks.
The 26-year-old made his big league debut with Miami last September. He appeared in eight games with one start, and struck out 13 in 13 1/3 innings.
"It's a fresh arm," Redmond said. "The thought right now is to use him in the bullpen as a long guy."
Tony Meale is a contributor to MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.