Phelps was acquired on Dec. 19, in a five-player trade that sent right-hander Nathan Eovaldi, first baseman Garrett Jones and prospect Domingo German to New York. Miami also received third baseman Martin Prado.
Phelps, 28, offers big league experience and flexibility. The right-hander also has Super Two status, meaning he is arbitration-eligible.
Phelps, Brad Hand and Aaron Crow are candidates for the fifth spot, and all three can be used in relief.
"We just wanted another versatile piece," Marlins president of baseball operations Michael Hill said. "We're going to give [Phelps] an opportunity to compete and be one of 12 with our club.
"I'm looking forward to the competition. It should be a great competition with Phelps and Hand and Crow. It should be interesting to see how it unfolds, but we love the versatility."
Miami's top four, in no particular order, are Mat Latos, Henderson Alvarez, Jarred Cosart and Tom Koehler.
Per Haren's wishes, Miami may seek to trade him to a West Coast team.
No matter how the rotation sorts itself out in Spring Training, the club made a deliberate effort to assemble as much depth as possible. History shows it's needed. In 2014, Miami used 13 starters.
Phelps promises to play a big role, as a starter or reliever. With New York, he was 5-5 with a 4.38 ERA in 32 games (17 starts).
The Marlins also know they have an ace in the wings ready to join the rotation around midseason. That's when Jose Fernandez should be recovered from his Tommy John surgery.
"We know the big man, Jose, is coming back in June or July and we wanted to give ourselves as much flexibility as possible in our staff," Hill said.