The Yokohama Bay Stars announced on February 22 that Koya Mizuno had surgery to clean his right shoulder at a hospital in Yokohama. There were no complications during the procedure.
[UPDATE 2/23 @ 1:14pm]
Mizuno could require about six months for a full recovery. He wanted to avoid surgery and spent the off-season rehabbing the shoulder, but decided to go under the knife because he still felt pain while throwing bullpen sessions in January.
Source: Nikkan Sports 2/23/2018