Dean Green suffered a strain in his left calf during fielding practices on April 23. He could end up missing quite a bit of time.
The Yakult Swallows signed Green as insurance for Kazuhiro Hatakeyama. However, due to roster restrictions, he was stuck at Ni-gun. There were plans to call Green up when they decided to take Ross Ohlendorf off the active roster on April 23, but those plans changed when he suffered the injury on the same day.
Source: ZakZak 4/28/2017
[UPDATE 5/23 @ 6:42pm]
Green could join the Ichi-gun team during Inter-league play. He already started taking live batting practice and was slated (as of May 22) to play in his first Ni-gun rehab game today (pinch-hitter).
If Green makes the active roster, Preston Guilmet might be the odd man out.