"After we got the first MRI, we sat down with [Ahmad] and he said, 'Listen, as long as it's not getting worse, we're good,'" Phelps said after New York's 8-7 victory. "The ligament was clean, everything was good. Today was what really set me off is today was the day it was worse. I had to say something. We'll find out what it is tomorrow and go from there."
Phelps was hit hard by the Red Sox on Sunday, surrendering five runs on six hits in two innings, including a two-run Dustin Pedroia homer. Girardi suggested that Phelps could miss his next turn in the rotation.
"I believe he's going to pitch again [this year]," Girardi said. "I don't know if it'll be his next turn, but I believe he's going to pitch again. He had the MRI before and it came back clean. He might need a little time-out."
Phelps started the year in the bullpen before being moved to the rotation due to injuries; Phelps is 5-5 with a 4.24 ERA in 26 games (17 starts) this season.
If the Yankees need a spot start, they could turn to right-hander Esmil Rogers, who hurled three scoreless innings in his Yankees debut on Sunday and had been starting at Triple-A Buffalo in the Blue Jays' chain, where he was 2-2 with a 3.14 ERA in 12 games (seven starts).
The Yankees could also consider giving a start to left-hander David Huff, right-hander Chase Whitley or Triple-A right-hander Bryan Mitchell.
"It's frustrating. I felt like coming into this start I was throwing the ball the best I had in my career consistently," Phelps said. "Anytime we're talking about a DL stint, it's frustrating. I was really enjoying helping the team win ballgames. It's frustrating. We're playing better baseball and I feel like we're about to go on a run."
Bryan Hoch is a reporter for MLB.com. Follow him on Twitter @bryanhoch and read his MLBlog, Bombers Beat. This story was not subject to the approval of Major League Baseball or its clubs.