Phelps was removed from his last start after allowing three runs in just 2 1/3 innings against the Cardinals on Sunday. The following day, he was placed on the disabled list with a forearm strain, but an MRI on Monday showed a stress fracture in the radius bone of his right arm.
Manager Dan Jennings was unsure if Phelps, 28, would be fully recovered by next year's spring training.
“It's unfortunate,” Jennings said. “[The injury] is in a very strange spot on the top of his forearm.”
Phelps, a four-year veteran, has a 4.50 ERA this season in 19 starts and four relief appearances. He fared better as a starter, despite his 4–8 record. His ERA as a starter is 4.39, compared with 6.75 as a reliever.
Rookie 23-year-old Kendry Flores will take Phelps's slot in the rotation after posting a 2.35 ERA in six relief appearances.
The Marlins have a 49–70 record, third worst in the majors.
- Erin Flynn