"It's unfortunate," manager Dan Jennings said. "[The injury] is in a very strange spot on the top of his forearm."
The recovery time for Phelps is yet to be determined, according to Jennings, who wasn't sure if Phelps would be ready to go come Spring Training.
Rookie Kendry Flores will take Phelps' spot in the rotation. The right-hander has posted a 2.45 ERA in five games out of the bullpen for Miami this season, allowing nine hits over 7 1/3 innings.
Phelps' injury is yet another to a Marlins rotation that will now consist of three rookie starters. Flores will join Adam Conley, who started on Tuesday against the Brewers -- one day after Justin Nicolino fired 6 2/3 innings in a victory over Milwaukee.
The Marlins have now used 12 different pitchers who are 25 years old or younger this season, the most in the big leagues.
Other injury updates
• Outfielder Christian Yelich, who was placed on the 15-day DL on Saturday (retroactive to Aug. 10) also underwent an MRI exam on Monday and is fine, according to Jennings. Yelich suffered a right knee contusion.
• Right-handed pitcher Jarred Cosart (vertigo resulting from an inner ear disorder) pitched a successful simulated game on Monday, and he will make a rehab start on Saturday with the organization's Rookie level affiliate of the Gulf Coast League.
• Right-handed pitcher Jose Fernandez (right biceps strain) will begin throwing from 75 feet. Fernandez was able to throw up to 60 feet on Monday.
• Reliever Carter Capps (right elbow strain) will throw a bullpen session on Thursday, and reliever Andre Rienzo (left knee laceration) will throw one on Wednesday. Rienzo will also throw live batting practice before the end of the week.
Brandon Curry is an associate reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.