Hughes said he first felt the strain, characterized by the club as "mild," faintly after his aborted start on April 24 at Chicago, when he hurled two scoreless innings but did not return following a rain delay. He will be sent for a MRI in New York on Thursday.
"It's frustrating, but I've just got to deal with it," Hughes said. "There's not much else I can do. Things are not going so well right now and add this on top of it, it's just frustrating."
While he said his side sessions between starts were not affected, Hughes was rapped for six runs and eight hits in 3 2/3 innings on Tuesday by Detroit, after which point he mentioned the injury to the club's training staff.
Yankees manager Joe Girardi said that he watched game tape of Hughes' start after learning of the injury and spotted a point in the fourth inning where Hughes sat on the bench, let out a deep breath and appeared to wince. Girardi said that he did not believe Hughes' performance had been affected since the club's radar gun had him touching 94 mph on Tuesday.
"Physically, he's looked great to me, so I was shocked when I heard it from the trainers," Girardi said.
Earlier Wednesday, Girardi grew testy in a pregame meeting with reporters in the dugout, repeating several times, that "Hughes is in the rotation right now."
When pressed and asked if Hughes was expected to pitch on Sunday against the Mariners, Girardi said, "That's the plan. There's no one else here."
Cashman said that the Yankees expect Hughes' recovery process to take more than 15 days, with a Minor League rehabilitation assignment necessary at some point.
As a point of comparison, Yankees outfielder Bobby Abreu suffered an oblique strain in Spring Training last year and did not come out of an offensive funk until June, and the team wants to be arguably even more careful with Hughes, whom they view as a prized prospect.
"You've got to make sure with the torque he has and the twisting he does, that he doesn't feel it at all," Girardi said.
Hughes, 0-4 with a 9.00 ERA in six starts this month, was next scheduled to start against the Mariners on Sunday, but said that the time off may do him well. He said the injury feels like a "dull pinch."
"I've got to sit and wait anyway, so hopefully I can clear my head of all the negative stuff that's going on," Hughes said.
Cashman said that Sunday's start will instead go to right-hander Darrell Rasner, who is 4-0 with a 0.87 ERA and was named the International League's pitcher of the week for the period of April 21-27.
"The guy who's pitching the best down there is Darrell Rasner, so that's the guy we're going to tap," Cashman said.
Rasner was in Spring Training with New York as a non-roster invitee and narrowly missed making the Opening Day roster as a long reliever.
Yankees left-hander Andy Pettitte said that the disabled list assignment could be a "blessing in disguise" for Hughes, who also missed a good chunk of last season with a strained left hamstring suffered on May 1 and a sprained left ankle he turned while performing conditioning exercises in Tampa, Fla., on May 25.
"He's just got to make the best of it and get it healthy, and try to get his stuff together," Pettitte said.