He 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.
The Yankees have already lost four starting pitchers to injury this season. Ivan Nova torn 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.
Phelps, who was added to the rotation when Pineda was suspended for 10 days for using pine tar on the ball on April 24, had pitched well recently, winning four of his last five decisions.