Promoting Conley and Peters are the corresponding moves after Junichi Tazawa was designated for assignment and Tyler Cloyd was optioned to New Orleans following Thursday's 7-0 loss to the Dodgers at Marlins Park.
"It's nice to have the lefties out there," Marlins manager Don Mattingly said. "They are pretty much starters. We still look at them as starters, but we will keep trying to do what's best for the guys in building their development."
With the Braves featuring tough left-handed hitters like Freddie Freeman, Nick Markakis and Ender Inciarte, the Marlins now enter the weekend series in Atlanta with three southpaws in the bullpen. Until recently, Miami didn't have a lefty reliever.
"We talked about it, for not just this series, but in general," Mattingly said. "When you face Atlanta, they're throwing three or four lefties at you. You really want to be able to turn guys around with their lineup, a little bit. If you don't have a lefty out there, if you have a lefty starter, as soon as you switch, they can go all left-handed on you. They can stack all their left-handed hitters together without any fear of you bringing in a lefty."
Peters and Jarlin Garcia -- the Marlins' third lefty in the bullpen -- are on innings limits this season. Over the course of the year, the Marlins are expected to shuffle starters in and out of the rotation to keep their innings down.
Garcia is in the bullpen now because the organization is managing his innings, and eventually he will move back into the rotation. Peters and Conley are starter possibilities, but their long-term roles could be in relief.
"We're not giving up on anything," Mattingly said. "It's a matter of opportunity right now."
Conley, who turns 28 on Thursday, went 2-4 with a 5.18 ERA in 40 innings. He last pitched on Monday, going eight frames in a win for New Orleans.
After a slow start, Conley has gotten progressively sharper with each outing. In his past five starts, he has a 1.93 ERA in 32 2/3 innings with 17 strikeouts and six walks. At the start of the season, he scuffled with a 19.64 ERA in 7 1/3 innings across his first three starts.
Conley has been in and out of the Marlins' rotation since 2015, and his best big league season came in '16, when he finished 8-6 with a 3.85 ERA in 25 starts. Last year, he split time in Miami and New Orleans. With the Marlins, Conley posted an 8-8 record with a 6.14 ERA in 102 2/3 innings. He also threw 62 1/3 frames for the Baby Cakes, compiling a 3-3 record and a 5.49 ERA.
"I know for right now, I'm in the 'pen," Conley said. "That's all I know. So, I'm going to do that the best I can. I'm super grateful. My arm works well that way. Getting up and getting hot quick and bouncing back. Those haven't been an issue for me in the past."
Peters, 25, has been up and down a few times already this year, appearing in six games (five starts) and going 2-2 with a 6.31 ERA. His last appearance with the Marlins was one inning of relief on May 2 against the Phillies.
At New Orleans, Peters had a 3.75 ERA in 12 innings over two starts. He logged 63 frames in the Minor Leagues last year.
"The move to put me in the bullpen, I'm 100 percent okay with," Peters said. "I'm obviously glad to have the opportunity to help this team win."
Worth noting Right-hander Brett Graves (left oblique strain) threw three innings in extended spring on Thursday, and he will toss a bullpen session on Sunday. Odrisamer Despaigne (right forearm strain) will pitch on Monday in a rehab assignment game with Class A Advanced Jupiter. Garrett Cooper (right wrist contusion) has started hitting and field progression.