Shun Kubo failed in his first defense of the WBA super bantamweight title on Sunday, losing to American Daniel Roman in a ninth-round technical knockout.
Kubo struggled with Roman's power at Shimadzu Arena Kyoto, where the southpaw was knocked down in the seventh and eighth rounds before the fight was stopped in the ninth.
Shinsei Gym chief Masato Yamashita said Kubo, who suffered his first career defeat in 13 fights, was unable to move his feet after the sixth round, with his legs cramping up when the bout ended.
"I want to build it back with him again," Yamashita said of the 27-year-old Kubo. "He had never lost before so the damage isn't just physical."
Kubo won the belt in April after defeating Venezuela's Nehomar Cermeno.
The 27-year-old Roman said the fight was "tough" but found his rhythm in the seventh round as he improved his record to 23 wins, including nine knockouts, with two losses and a draw.