Masataka Yoshida has had problems with his lower back since his rookie year. The media learned on November 8 that there is an increasing chance that he goes under the knife to treat the condition this winter, particularly if they can time the procedure so that he can still be ready for opening day.
Source: Sports Hochi 11/9/2017
[UPDATE 11/22 @ 3:10pm]
The media learned on November 21 that Yoshida decided to have surgery on his back this winter. He should be fine for spring camp next year.
[UPDATE 11/23 @ 2:16am]
Yoshida had surgery at a hospital in Tokushima. He will remain in the hospital for a few days.
Source: Nikkan Sports 11/22/2017