"Everybody is looking to get better any way they can, and I think we were kind of the same," Marlins manager Don Mattingly said Tuesday. "I know there were some other things going on at the Deadline, but knowing [Phelps] was there, obviously, we knew he has done this."
Switching roles is nothing new for Phelps, who has accepted change ever since he broke in with the Yankees in 2012.
In his first season with the Marlins in 2015, Phelps made 23 appearances, with 19 starts. His career ERA starting is 4.36.
A need for rotation depth was created when Wei-Yin Chen went on the disabled list with a left elbow sprain on July 25.
Miami felt it had solidified its rotation on Friday when it completed a seven-player trade with the Padres. Andrew Cashner and Colin Rea were brought in to fill two spots. But on Saturday, Rea worked just 3 1/3 innings before suffering a right elbow injury.
On Monday, Rea was swapped back to San Diego for pitching prospect Luis Castillo, who was also included in Friday's trade.
Phelps, who threw a bullpen session on Tuesday at Wrigley Field, views the transition back to starter as doing his part in helping the team reach the postseason.
"Absolutely," he said. "We're here for one reason, and it's to not to go out and experiment on stuff. We're here to try to get to the playoffs and win the World Series. So it's kind of all hands on deck at this point."
Phelps is in the process of building up his pitch count. In relief of Rea on Saturday in an 11-0 win over the Cardinals, Phelps threw 2 2/3 innings, and picked up the win.
"We've talked about it," Phelps said of going back to starting. "There will be a pretty good plan in place of getting it done."
As a starter, look for Phelps to utilize more pitches. Relievers can often get by with two pitches, but in the rotation, he promises to throw more curveballs and changeups.
Phelps, according to Fangraphs, is throwing 66.1 percent fastballs, 22.7 percent cutters, 10 percent curveballs and 1.2 percent changeups. In 2015, when he was mostly starting, he threw 60.2 percent fastballs, 17.8 percent cutters, 14.5 percent curveballs and 7.6 percent changeups.
Miami's bullpen also has the depth to allow Phelps to start without sacrificing a power arm late in games.
Kyle Barraclough, Fernando Rodney and closer A.J. Ramos have been handling the seventh, eighth and ninth innings. Plus, right-handers Nick Wittgren and Brian Ellington have shown promising signs, as has lefty Mike Dunn.
"We had already made the switch already to Claw in the seventh, so we already had seven, eight, nine locked down," Mattingly said. "Phelpsy was in that, six-innings, one-plus. It was a pretty easy transition for us."
• First baseman Justin Bour (high ankle sprain) is scheduled to continue his rehab assignment Tuesday and Wednesday, and if that goes well, he could be reinstated on Friday for the series opener at Colorado.