NEW YORK -- The Marlins are bracing to be without lefty reliever Mike Dunn for longer than initially expected.
Dunn experienced a setback in his bullpen throwing session on Sunday, and he is going to have his strained left forearm examined by team physician, Dr. Lee Kaplan.
"He didn't come out very good from his 'pen," manager Don Mattingly said. "He felt a little grab in there again. So, we're going to slow him down, and I think he's going to talk with Dr. Kaplan again and see where it goes."
Dunn opened the season on the disabled list, retroactive to March 25, which meant the earliest he could have been reinstated was this past Saturday. However, the initial hope was he would miss the first road trip and be ready for when the Marlins open a series on Friday against the Braves.
Right now, there is no timetable for a return.
"They kind of said he's going to be down a few days," Mattingly said. "We'll find out once he talks to the doctor, and we'll go from there. So not great news for him."
An initial MRI didn't show any structural damage. But the 30-year-old southpaw has some inflammation, and the club is being cautious about rushing him back.
Dunn has been one of the most durable and dependable Marlins relievers in club history. Since being obtained from the Braves as part of the Dan Uggla trade in 2011, Dunn has appeared in 354 games, second only to Braden Looper's 368.

Piecing together the bullpen has been a challenge for Mattingly. The club was already dealing without Carter Capps, who had Tommy John surgery early in Spring Training.
Craig Breslow and Chris Narveson are the two left-handed options on the staff. Right-hander David Phelps, who has been mostly a long reliever, is now in the mix for a late-inning role. Bryan Morris is another eighth-inning candidate, setting up closer A.J. Ramos.
"We've started the season like this," Mattingly said. "Obviously, Mike is a guy we're counting on. We're just not going to be able to count on him yet.
"We'll use a combination of guys. We'll just have to use a combination of guys back there."