Ryosuke Hirata has had problems with his left hip since the end of March. He was taken off the active roster on April 5. He is in pain and has trouble walking. He will spend the next few days resting.
Source: Chunichi Sports 4/5/2016
[UPDATE @ 10:18pm]
Hirata had difficult walking due to pain on April 4. He felt better this morning and believed he could play through the pain, but when he actually started moving around on the field during practices, he realized the pain was too much.
Hirata had similar problems last May and missed ten games. He was told it was groin pain syndrome.
Source: Tokyo Spots 4/5/2016