Tetsuya Yamaguchi was taken off the active roster on May 22. While no specific announcement was made, he was likely removed due to an injury of some sort. The only thing Yoshinobu Takahashi told reporters was that it would be impossible for Yamaguchi to pitch for at least ten days.
[UPDATE 7/17 @ 5:09pm]
Yamaguchi is scheduled to pitch in a batting practice scrimmage on July 19.
Source: Sponichi 7/17/2017
[UPDATE 7/29 @ 11:56am]
Yamaguchi is slated to pitch in his first rehab game at Ni-gun today (July 29).
Source: Nikkan Sports 7/29/2017
[UPDATE 7/29 @ 10:51pm]
Yamaguchi started a Ni-gun game against Yakult today and recorded a shutout inning. He faced three batters, threw twelve pitches, did not allow any hits, struck out one, and topped out at 138km/h.
Yamaguchi might be ready to return to the active roster some time around mid-August.
Added additional performance information on July 30 at 10:48pm.