Tatsuyoshi Tsutsumi stepped down as general manager of the Yomiuri Giants and was replaced by his special assistant Yoshitaka Katori on Tuesday.
The 60-year-old Katori, who was a relief pitcher for the Giants and the Seibu Lions and also served as the head coach for Yomiuri, becomes the first former NPB player to assume the franchise's GM role.
"I'm a bit surprised that an offer like this has come my way all of a sudden," Katori told a news conference. "As a former Giant, I'll take it in good faith and do the best job I can."
The Giants made several big offseason acquisitions, including free-agent pitchers Shun Yamaguchi and Masahiko Morifuku and outfielder Daikan Yoh. Injuries have sidelined Yamaguchi and Yoh.
Yomiuri had a franchise-worst losing streak of 13 games end on Friday. The Giants (24-35 overall), who are in fifth place in the Central League, then lost two more games over the weekend.
Tsutsumi, 51, became GM in May 2015.
"I am stepping down because we have let our fans down and that responsibility rests with me. I was not good enough," Tsutsumi said in a statement. "I wish the team nothing but success in the second half of the season under new management."
