Ryosuke Hirata was taken off the active roster today. The Chunichi Dragons told the media it was due to discomfort in his right knee. He has not been examined at a hospital.
Added additional information on 6/19 @ 1:41pm
[UPDATE 6/21 @ 11:55am]
Hirata was examined at a hospital on June 20. He wore a brace when he visited Nagoya Stadium after his examination. He told reporters he still felt some pain when moving his right knee in certain directions, but was otherwise okay. He also told reporters he had his knee drained and need to keep the brace on for a couple weeks. He could end up missing the first-half of the season.
[UPDATE 6/21 @ 9:29pm]
Hirata hurt his right knee when he slid head-first into home plate during a game against Seibu on June 17. He has been good about avoiding head-first slides because he hurt his right shoulder three or four times. He had to be told by Hiromitsu Ochiai not to do it. But in the game against Seibu, he let his emotions get the better of him. He has no regrets, since it happened in the heat of the moment, but he also never wants to do another head-first slide again.
Source: Tokyo Sports 6/21/2017