"Don't count on him pitching in New York for at least six weeks," Cashman said in a telephone interview. "It could be sooner, but it's safer to predict we will be without him for six weeks."
Sabathia has been on crutches as he recovers from a cortisone and stem-cell injection in his inflamed knee. The procedure was administered by Dr. James Andrews in Birmingham, Ala., late last week.
Cashman said the treatment was administered to treat degenerative changes in Sabathia's knee, specifically the breakdown of cartilage in the hurler's landing leg.
"He had the stem-cell knee injection, and it sounds like they have a better success rate the longer you allow it to work," Cashman said.
Cashman said the plan is for Sabathia to shed his crutches Monday and begin therapy in a pool on Tuesday. Ideally, he would progress to throwing on flat ground, mound work, Minor League rehab games and then a return to the Bronx.
Sabathia said he had been dealing with discomfort in his knee, and that it had been an issue in the late innings of a May 4 start against the Rays at Tropicana Field. He was sent for an MRI after surrendering three homers on May 10 against the Brewers at Miller Park. Sabathia is 3-4 with a 5.28 ERA in eight starts this season, surrendering 10 home runs in 46 innings.
With Sabathia, Ivan Nova (Tommy John surgery) and Michael Pineda (upper back muscle strain) on the disabled list, the Yanks have lost 60 percent of their Opening Day starting rotation. Nova is out for the season, while Pineda is expected to return in June.
Because of the injuries, the Yankees have bumped Vidal Nuno and David Phelps from the bullpen to the rotation. Chase Whitley was promoted from the Minors and made his big league debut in last week's Subway Series against the Mets.
Cashman said it "doesn't matter" if he is comfortable with the rotation as is.
"This is what we've got and we're going with, and we've been doing it, it seems like now, for quite some time," Cashman said. "Obviously we do pursue external options, but the time of year will pretty much dictate that you're staying in-house for quite some time."
The Yanks are encouraged that Pineda is making progress and should be able to help, having contributed to the early success of the rotation before serving a 10-game suspension for excessive pine tar and sustaining an injury while tossing to remain sharp.
Pineda, who has thrown bullpen sessions of 20 and 30 pitches, is traveling to Tampa, Fla., to continue his rehab. He'll advance to batting-practice sessions, and then Minor League rehab games.
In the event that the Yankees needed another starter quickly, Cashman said, he believes the internal choice would be to promote Adam Warren out of his relief role. Dellin Betances, who has excelled in a setup role, is not a candidate to start.
"[Warren] would be the next guy, in my opinion," Cashman said, adding, "We've had internal discussions on Warren, not Betances."
Cashman said he has remained engaged with the trade market, but little has materialized, in part because the Yanks are not alone having sustained pitching injuries this season. Cashman said that teams may be more inclined to change their thinking after the First-Year Player Draft in June.
"It's a very complex situation," Cashman said. "The scarcity of pitching is real, and so we will continue to give opportunities internally until anything that presents itself externally -- and that we can get our hands on -- exists."