General manager Brian Cashman released an update early Wednesday evening, revealing Chamberlain's diagnosis to club officials in Texas. Chamberlain will spend approximately the next week resting in New York, before commencing a throwing program shortly thereafter. He will not rejoin the team in either Anaheim or Minneapolis.
The Yankees recalled reliever Chris Britton to fill Chamberlain's roster spot and announced plans to recall Ian Kennedy for a start on Friday.
"You have to move on," Yankees manager Joe Girardi said. "That's the only thing you can do in this case."
Chamberlain left Monday's start against the Rangers after 4 2/3 innings with stiffness in his right shoulder, complaining of a "grabbing" sensation in his deltoid muscle. The Yankees flew him to New York early Tuesday morning for an MRI with team physician Dr. Stuart Hershon, and then to Florida on Wednesday for a second opinion from Dr. James Andrews.
The diagnosis was right rotator cuff tendinitis, the same injury that sidelined catcher Jorge Posada for the entire month of May. Posada eventually returned but never regained full strength in his throwing shoulder, undergoing season-ending surgery on a right shoulder strain last week.
The Yankees can only hope that Chamberlain won't suffer a similar fate.
"To be quite honest with you, you can't sit here and hang your head about it," shortstop Derek Jeter said. "Hopefully, it's not something that goes for an extended period of time."
Chamberlain, 23, was 4-3 with a 2.63 ERA, and 3-1 with a 2.76 ERA since becoming a starter in early June. He joins fellow Yankee starters Chien-Ming Wang and Phil Hughes on the disabled list.
Kennedy also spent part of this season on the disabled list but recovered and has been pitching for Triple-A Scranton since early June. There, he has posted a 3-0 record and 1.33 ERA over his past four starts, giving the Yankees reason to believe he might be an able sub for Chamberlain. Kennedy posted a 1.89 ERA over three starts last September, providing further hope that he might excel on Friday.
"Obviously, he wouldn't make that start if Joba wouldn't have gone down," Girardi said. "But we're expecting him to come up and pitch the way he did in September."
Though Dan Giese was originally slated to start on Friday, the Yankees pushed him back to Saturday so that they could use him in relief at some point this week.
Anthony DiComo is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.