Tanaka has struggled through a difficult campaign, as he is 8-10 with a 4.92 ERA and 1.28 WHIP with 141 strikeouts in 133.2 innings pitched.
In his last outing, the Japanese righty lasted just four innings against the Toronto Blue Jays, surrendering two hits, five walks and three runs, two of which were earned.
Most of Tanaka's struggles came early in the year, as he's bounced back with a combined ERA in June and July of 3.88.
Those worries were validated in 2014 when Tanaka suffered a partially torn ulnar collateral ligament. While he avoided Tommy John surgery, he was limited to 20 starts as a rookie. In 2015, Tanaka experienced soreness in his wrist and forearm that kept him out for over a month.
The Yankees have been ahead of schedule in terms of their return to World Series contention despite Tanaka's struggles.
His injury comes at a bad time for the Yanks since they placed veteran lefty CC Sabathia on the DL on Friday (retroactive to Wednesday).
Rookie Jordan Montgomery will replace Sabathia in the rotation, and options to step in for Tanaka include Bryan Mitchell, Caleb Smith, Luis Cessa and highly touted prospect Chance Adams.