On August 24, 2017, Kozawa debuted in the Pacific League against the Saitama Seibu Lions,[2] and pitched two games in the Pacific League. In June 2018, Kozawa hurt his neck and back and spent the season rehabilitating.[3] On November 20, he left the active roster and signed as a developmental player.[3][4]
In 2019, Kozawa played in the Western League of NPB's minor leagues, and pitched in eight games and recorded an ERA of .193.[5] In 2020, Kozawa played in the Western League, and pitched in six games and recorded an ERA of .5.51.[6] On November 4, Hawks has released him.[7]
^In the NPB, in addition to the 70 registered players under management (active roster of players), each team can have developmental squad players for the purpose of improving their baseball skills and training manners. They aim to be promoted to registered players under management within the three-year contract period (Non-tender FA players and contract expired players renew their contract every year).