Karstens, a 24-year-old rookie, was making his second start of the season for New York after missing the Opening Day roster due to right elbow tendinitis. His first pitch was a fastball to Red Sox leadoff hitter Julio Lugo, who ripped a line drive off the hurler's right leg, sending Karstens into a pained crumple on the mound.
"You start the season on the DL, and come back, they give you another shot to pitch, and the first pitch of the game the guy hits a line drive off my leg," Karstens said. "It's a long season though, so I've got to come back and come back strong."
New York's medical staff immediately attended to Karstens, who said that the injury hurt but insisted that he could remain in the game. Karstens threw several warmup pitches and faced one more batter, Kevin Youkilis, throwing five pitches before the Boston first baseman reached Karstens for a single to left field.
"I think at first, he was scared, because it hurt," Yankees manager Joe Torre said. "He was walking around and said, 'I can feel it, but it's OK.' But we didn't want him throwing with his arm and not pushing off with the leg that he hurt."
Citing those concerns, the Yankees removed him from the game in favor of left-hander Kei Igawa, who was made available for long relief efforts in this series after a poor outing on Monday against the Rays. Igawa threw six shutout innings of two-hit relief and picked up his second Major League victory.
"We've had a lot of stuff," Yankees general manager Brian Cashman said. "You couldn't print what was going through my head and coming out of my mouth at the time that it happened, seeing [Karstens] in such pain on the field. You worry about him first and foremost, but you also think about what's going on with your club. For Igawa to come in and do what he did, it was a huge lift."
Karstens underwent X-rays at Yankee Stadium, which revealed the fracture, and was later taken for additional X-rays and a CT scan at Columbia Presbyterian Hospital in New York before returning to Yankee stadium sporting a soft foam cast.
Team physician Dr. Stuart Hershon said that Karstens should expect to be capable of pitching again this season, though both Hershon and Cashman declined to give a timeframe.
The Yankees will place Karstens on the 15-day disabled list, but as of Saturday evening, Cashman said the organization was still discussing which Minor League pitcher would be promoted to New York.
Bryan Hoch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.