According to Yankees manager Joe Girardi, Phelps has suffered from the inflammation for "three or four weeks" and had an MRI, that came back clean, after a start against the Cincinnati Reds on July 18 in which he pitched well and won a 4-3 game.
"We thought it was something we could manage, and he was managing," Girardi said. "He was pitching well. It was just inflammation. But today, for whatever reason, it bothered him."
Girardi said Phelps will be evaluated by team doctors after the Yankees return to New York for a seven-game homestand that begins Monday night.
Phelps was ineffective in his brief stint, allowing three first-inning runs on four hits, and two more in the second on a home run by Dustin Pedroia. He completed the inning but was replaced in the third by Chase Whitley.
"Last couple of starts it would loosen up and I'd be good to go," Phelps said. "Today it just didn't loosen up."
The Yankees have already lost four starting pitchers to injury this season. Ivan Nova tore his ulnar collateral ligament in a game against Tampa Bay on April 19; Michael Pineda suffered a lat strain that has kept him on the disabled list since May 6; CC Sabathia suffered a knee injury that shut him down on May 11 and eventually led to season-ending surgery, and Masahiro Tanaka has been on the DL since July 9 with a partially-torn UCL.
Girardi said he didn't expect the injury to end Phelps' season, although he strongly indicated the right-hander would miss at least one start.
"I believe he's going to pitch again," Girardi said. "But he might need a little time out."