TORONTO -- The Yankees announced Tuesday afternoon that ace righthander Masahiro Tanaka underwent arthroscopic surgery at the Hospital for Special Surgery in New York to remove a bone spur from his right elbow.

Recovery time is about six weeks, with Tanaka expected to begin a throwing program afterward and to be ready for the start of spring training.

According to the club, the spur "was pre-existing, dating back to his pitching career in Japan."

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GM Brian Cashman said it was something the Yankees were fully aware of when they made their $175-million investment in Tanaka in January 2014.

"It was asymptomatic," Cashman said by phone yesterday.

Cashman added: "He wasn't receiving any treatment on it" with his team in Japan, the Rakuten Golden Eagles.

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"The majority of the pitching population is pitching with bone spurs that are asymptomatic."

But, unbeknownst to the Yankees, Cashman said, Tanaka started feeling discomfort from the spur this season, even as he went 12-7 with a 3.51 ERA. The 26-year-old passed his exit physical but, through his agent Casey Close, about week later contacted the Yankees. After a sit-down, a determination was made for the surgery, which Cashman characterized as "not complicated."

"It was a surprise to us," said Cashman, "because he had no complaints and had a pretty good year."

The first part of that was addressed in the meeting because the Yankees continually asked Tanaka how he was feeling.

"He was telling us he was fine," Cashman said. "There wasn't anything conveyed all season."