Tomoya Hoshi was taken off the active roster on September 8 with tightness in his right elbow.
Source: Sanspo 9/8/2017
[UPDATE 9/15 @ 10:24pm]
The media learned on September 14 that Hoshi was examined at a hospital in Tokyo on September 13 and diagnosed with a fatigue fracture in his right elbow. He will not be allowed to do any throwing for a while.
Source: Sanspo 9/15/2017