Cashman said that Sabathia will undergo a cortisone and stem-cell injection on Thursday at Andrews' practice in Birmingham, Ala., and that there is no immediate timetable for his return. Cashman mentioned that several big leaguers, including Carlos Beltran, Andruw Jones, Hideki Matsui, Raul Ibanez and Randy Johnson, have had success with similar treatments.
"His knee stability is fine, so there's no ligament damage or anything like that," Cashman said. "His knee is stable, but he does have some degenerative changes. What does that mean? He has some cartilage breakdown that is occurring. We have current players, and we've had past players, that have dealt with this in the past."
"It's just basically getting everything to calm down in his knee," manager Joe Girardi said. "You hope it's just the 15 days for CC."
Sabathia, who is 3-4 with a 5.28 ERA in eight starts this season, said that he first felt the discomfort during a May 4 start against the Rays.
His absence marks yet another pitching injury for the Yankees, who lost Ivan Nova for the year to Tommy John surgery and will be without Michael Pineda until at least June with a Grade 1 strain of the teres major muscle behind his pitching shoulder.
David Phelps and Vidal Nuno already have been promoted from the bullpen to the rotation, with right-hander Chase Whitley set to take Sabathia's place in the rotation for Thursday's Subway Series finale against the Mets. Whitley will be added to the roster on Thursday.
"The big thing for me with him is, as much as he can, he gets through that first inning to calm himself down," Girardi said of Whitley. "I don't care who you are and how highly touted you are, that first time you walk on a Major League field, there's a lot of emotions and a lot of adrenaline."
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.