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
[UPDATE 9/26 @ 8:04pm]
The Swallows announced today that Hoshi will have surgery at a hospital in Yokohama on October 12.
[UPDATE 10/14 @ 1:52am]
Hoshi had surgery on his right elbow on October 12. There were no complications.
Source: Sanspo 10/13/2017
[UPDATE 10/30 @ 3:56pm]
Hoshi will not do any throwing for the rest of the year. He is planning to start playing catch in January.