Dunn has struggled with command all season, at no time more than in the Marlins' series-opening 14-7 loss to the Giants. The southpaw made an appearance for Miami in the eighth and gave up four runs on three hits while walking two batters in an inning of work. He threw 38 pitches, 23 for strikes.
"I can't figure it out, this kid has a good arm," Marlins manager Ozzie Guillen said after the game. "His breaking ball is not working. His location is not the greatest location. ... That arm is gold. It's gold. You can throw 95 [mph], but if you can't throw the ball over the plate and your breaking ball is not working, you're going to get hit."
In 12 appearances for the Marlins, Dunn has walked 10 and struck out 17, and he has a 9.58 ERA in 10 1/3 innings.
Thursday marked the second time the Marlins have optioned Dunn this season. While the club hasn't announced who will be recalled to replace Dunn on the 25-man roster, there are a few likely candidates. With Randy Choate the only remaining lefty in the bullpen, Miami could opt to recall left-hander Dan Jennings.
If the Marlins feel comfortable with only one left-handed reliever, they could promote either righty Chris Hatcher or Sandy Rosario from the Zephyrs. Hatcher has 1.71 ERA with 17 strikeouts and five walks in 21 innings for New Orleans, while Rosario sports a 1.42 ERA in 19 innings of work, with 17 strikeouts and just two free passes.
Tom Green is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.