Sabathia, who has had a rocky start to the season and pitched poorly once again in Saturday's 5-4 loss to the Milwaukee Brewers, underwent an MRI on Sunday morning.
According to manager Joe Girardi, the test showed no structural damage but did show some fluid in the knee. Sabathia said his knee would be drained in New York on Monday by Yankees team doctor Christopher Ahmad, after which he might undergo a second MRI.
Sabathia said he's felt the injury in a couple of starts earlier this season but that it worsened Saturday night.
"I tweak it every now and again," he said. "But (on Sunday, in his previous start against the Tampa Bay Rays) I felt it pretty much in the later half of the game and then all of the game Saturday, so I thought I better say something."
Sabathia, who is 3-4 with a 5.28 ERA this season, did not take the loss Saturday night but allowed three long home runs and left the game with the bases loaded and one out in the sixth inning and the Yankees trailing, 4-3.
Because of an error by shortstop Brendan Ryan that prolonged the third inning, in which Jonathan Lucroy and Aramis Ramirez followed with home runs, Sabathia was charged with just one earned run.
"As of right now there's no meniscal tears. He's just got some fluid and he needs a little time off," said Girardi.
But Sabathia, who has three seasons plus an option year left on a contract potentially worth $96 million, has been hit hard all year and leads the majors in home runs allowed with 10.
"After the game last night, CC complained that his knee was hurting; his landing knee, his right knee," Girardi said. "He hadn't mentioned anything to us before that. He said a few previous starts, he would feel it on one pitch, but it was something that he's dealt with for a while, so he didn't have a need to say anything to us. Yesterday it got bad and it swelled up after the game, so he had an MRI."
Sabathia said he was reluctant to report the injury because of all the injuries the Yankees have already suffered to their starting rotation.
"It's tough," he said. "That's why I didn't want to say anything, but I think I was doing more damage to the team than helping the team by trying to hide it."
The Yankees' rotation had already been depleted by injuries to Ivan Nova, who is out for the season after undergoing Tommy John surgery, and Michael Pineda, who is on the DL with a strained back muscle and is expected to be out at least another month.
Nova and Pineda have been replaced in the rotation by lefty Vidal Nuno and righty David Phelps, who is starting Sunday's series finale against the Brewers.
"Obviously it tests your depth and it stretches you," Girardi said. "The two guys that we brought in so far have done a pretty decent job. But we're going to be tested further now."
Girardi said he had not noticed any physical problems while Sabathia was pitching on Saturday, but the left-hander had uncharacteristically bounced some fastballs, and failed to cover first base on an infield hit in the sixth inning.
"I don't really see him bounce fastballs very often, but I didn't think much of it," Girardi said.
The Yankees could provide no concrete prognosis for how long Sabathia will be out.
"We're hoping it will only be 15 days," Girardi said. "Obviously he's got to see [team doctor Chris] Ahmad tomorrow. Because there's no meniscal tears -- I mean, that's what our fear was because that's what he's had in the past -- we're hoping it will only be the two weeks. We just have to get it to calm down."
Sabathia said the knee became swollen after last Sunday's start, and again Saturday night.
"I didn't know what was making it swell up," he said. "It got pretty big on me after the start Sunday and after the start last night. So of course, I was getting a little nervous."
The Yankees recalled right-handed pitcher Matt Daley, a reliever, to their roster to take Sabathia's place, but his rotation spot is likely to be assumed by right-hander Alfredo Aceves, who took the loss on Saturday.
CC Sabathia Placed On Disabled List
ESPNNewYork.com's Wallace Matthews discusses the knee injury that has sent Yankees ace CC Sabathia to the DL.