Sources: Naoya Inoue injured, bout vs. Stephen Fulton ppd.

Naoya Inoue suffered an injury in training and his May 7 fight in Yokohama, Japan, vs. Stephen Fulton for the unified 122-pound championship has been postponed, sources tell ESPN.

The fight, which was set to be streamed on ESPN+ stateside, is being rescheduled for the summer in Japan, sources said. The nature of the injury is unclear, per sources, but the severity isn't deemed serious.

Inoue (24-0, 21 KOs) vacated his four 118-pound titles in January -- one month after he captured the undisputed bantamweight championship with an 11th-round KO of Paul Butler -- to chase titles in a fourth weight class. The Japanese star quickly closed a deal to fight ESPN's No. 1 boxer at 122 pounds: Fulton. The 28-year-old Philadelphian holds the WBC and WBO junior featherweight titles.

Fulton (21-0, 8 KOs) isn't simply the top fighter at 122 pounds; he's also one of the best regardless of weight. A fight featuring Fulton and Inoue is among the best matchups the sport can deliver.

Fulton, who has never fought professionally outside the U.S., told ESPN in June that he believed a fight with Inoue was "realistic" if Inoue moved up to 122 pounds before Fulton eventually jumped up to 126.

"I don't shy away from big fights," Fulton said then. "A lot of people wanna see [me vs. Inoue]; a lot of people wanna talk about that as well.

"Had I been more known like Inoue, I'd be on that [pound-for-pound] list."

Inoue, 29, is one of the most dynamic boxers in the world, a fighter with blinding speed who uses angles, punch variety and ring IQ to make quick work out of top contenders. He was involved in ESPN's 2019 Fight of the Year, in which he pushed through a broken nose and fractured orbital to outpoint Nonito Donaire.

Inoue left no doubt in the rematch last June when he scored a second-round TKO of the future Hall of Famer.

Against Fulton, Inoue will certainly meet his biggest -- and toughest -- challenge to date. Fulton is a career-long 122-pounder who can box from the inside and from the outside and often throws upward of 1,000 punches in his fights. Inoue, meanwhile, began his career at 108 pounds.

"The real battle starts from here," Inoue said in January.

Now, Inoue, Fulton and fans alike will have to wait a little bit longer for the highly anticipated matchup.