Hughes was placed on the 15-day disabled list on Wednesday with what was originally believed to be a right oblique strain. That diagnosis came from Yankees team physician Dr. Stuart Hershon, hours after Girardi originally defended Hughes' spot in the rotation in a sometimes-contentious exchange with reporters.
New information revealed on Thursday showed the stress fracture. Cashman said that Hughes will now be unable to perform any strenuous activity for four weeks, extending the time frame of the injury.
Cashman said that Hughes showed no discoloration on his side and that he did not recall how he may have suffered the injury, but was satisfied that Hughes is not hiding an injury, calling his demeanor "honest and forthright."
"There's no indication or any episode he can recall of an action that took place," Cashman said. "There was [not] some bang, a car door opening, a fall on a coffee table. We asked every question and there's no bruise. The first time he noticed it was from a cough a week ago."
Hughes said that he had racked his brain to think of any event in the last few months where he could have injured his right side, but had so far come up empty. He first felt the injury after a two-inning, rain-shortened start at Chicago on April 24.
"It wasn't like one specific pitch where I felt it," Hughes said. "It was one of those things where I woke up one morning and there was a little discomfort, but nothing major. After [Wednesday] night, there was significantly more discomfort."
Right-hander Darrell Rasner will be promoted from Triple-A Scranton/Wilkes-Barre to start against the Mariners on Sunday in Hughes' place, but the Yankees had not officially executed that move as of Thursday. New York planned on playing with a 24-man roster for their game against the Tigers.
"I'm disappointed for Phil and for us that he is not available to us for an extended period of time," Cashman said. "We will have to make adjustments because of that."
Hughes, the youngest pitcher in the Major Leagues, was 0-4 with a 9.00 ERA in six April starts.
"He had a great March and pitched well against Toronto [on April 3], and had two good innings against the White Sox," Cashman said. "In between, he's had bad starts. How much of that is because of the ribcage? I don't know. All I know is we've been searching for his fastball command. His velocity has been there. Ultimately, we're in a gray area where we don't know when this occurred and what necessarily caused it."
With Hughes scheduled to be re-evaluated in four weeks, the Yankees will have to send him to the Minor Leagues on a rehabilitation assignment, something they would have done anyway under the timetable of the original oblique strain diagnosis. Cashman said that the process to get Hughes back to New York in big league form would run until midsummer.
"Starting pitchers take longer than anybody, because they've got to build their arm strength up and their ability to go deep in games," Cashman said. "I think now we have a better idea why his fastball command has been off."
Hughes said he had no target date in mind for his return.
"I was told I could throw in a few weeks, and I'll try to get that process going as soon as I can," Hughes said. "Obviously, I want to be out there. I know our training staff is going to do what's right."
This marks the second consecutive season where Hughes has missed significant time due to injury. He strained his left hamstring while working on a no-hitter in his second Major League start last May 1 at Texas, and then suffered a setback on May 25, when he sprained his left ankle while performing conditioning exercises at the Yankees' Spring Training facility in Tampa.
Hughes would return to New York on Aug. 4 after five rehab starts, going 3-0 in September and logging the Yankees' only victory of their four-game American League Division Series against the Indians.
"I know he can pitch here, because I saw him do it last year in a pennant race, asking a lot of him," Cashman said. "He did well, as well as October in the playoffs coming in relief and securing a win [in Game 3 of the ALDS]. I know it's not the environment."
Hughes said that he was not concerned that he might be building a reputation as injury-prone.
"If I was constantly having shoulder or elbow problems, things of that nature, I'd be concerned," Hughes said. "It's two kind of freak things that happened. I think that's more just bad luck than anything."