It is unclear when the injury occurred, but Phelps' only start since Aug. 5 was Sunday at St. Louis. He lasted just 2 1/3 innings and was placed on the disabled list the following day.
"It's unfortunate," Jennings said. "(The injury) is in a very strange spot on the top of his forearm."
Phelps, who is 4-8 with a 4.50 ERA, will be replaced in the rotation by Kendry Flores. A right-hander, Flores has appeared in five games in relief and has a 2.45 ERA.
Flores becomes the third rookie starter for Miami, which has used 12 pitchers who are 25 or younger this season, the most in the majors.