Duncan, 28, was admitted last week and is no longer hospitalized, his agent, Barry Meister, and team general manager Brian Cashman said, according to the report.
The Yankees asked Duncan's family not to comment publicly on the situation, the Post reported.
Meister told the Post that Duncan was out of the hospital but referred further inquiries to the Yankees. Cashman declined to comment on why Duncan was hospitalized, the severity of his problem or his long-range diagnosis, the Post reported.
Duncan underwent hernia surgery 10 days after the end of the season, but it is not known if his hospitalization and treatment was connected to that surgery, the Post reported.
Duncan is being considered for an outfield or first base position next season. He hit .257 with seven home runs and 17 RBIs in 34 games after being called up from Triple-A Scranton/Wilkes-Barre in July.
Duncan had been scheduled to appear Monday night with fellow Yankees prospect Joba Chamberlain at an event in New Rochelle, N.Y., but did not attend.