Juan Diaz (shoulder) out 6 months

Former unified lightweight titleholder Juan Diaz tore the rotator cuff in his left shoulder during a sparring session earlier this month and will undergo surgery on Wednesday to repair it, manager Brian Caldwell told ESPN.com.

Diaz was training for a world title fight against Japan's Takahiro Ao. They were due to fight on May 1 at the Cosmopolitan in Las Vegas for the 135-pound world title that 2014 fighter of the year Terence Crawford will vacate before his April 18 bout with Thomas Dulorme for a vacant junior welterweight world title.

Diaz-Ao was due to headline the debut of promoter Top Rank's new series on cable network truTV, although the series has not been formally announced yet. Ao (27-3-1, 12 KOs), a 30-year-old southpaw from Japan and a former featherweight and junior lightweight titleholder, will instead face former two-time lightweight title challenger Raymundo Beltran (29-7-1, 17 KOs), 33, a Mexico native living in Tempe, Arizona, for the soon-to-be vacant belt on May 1.

Caldwell said Diaz was sparring when he injured himself throwing a left hook to the sparring partner's body.

"Juan was sparring with a southpaw and he threw the shot and winced in pain," Caldwell said. "He kept going for the rest of the round but when he came back to the corner he said he hurt his shoulder.

"We did conditioning work the next day and the shoulder was still giving him problems, so he went to the doctor the next day. He ended up seeing three doctors and all three concluded that, after looking at the MRI, he needs surgery. They said his rotator cuff is attached by a thread so it was inevitable that he would have had a complete tear if he continued to use it without getting surgery."

Caldwell said Diaz, known as "The Baby Bull," is likely to be sidelined for about six months.

Diaz (40-4, 19 KOs), 31, of Houston, won a world title in 2004, made seven defenses and unified three major belts before being upset by Nate Campbell in Cancun, Mexico, in 2008.

After suffering through a stretch in which he lost three of four fights, including two world title fights to Juan Manuel Marquez, Diaz retired in 2010.

He decided to come back in 2013 and has won five fights in a row to put himself in position for the title bout with Ao.

"I feel bad for Juan because of the effort and time he has put into his comeback. Everything was on track for him to get another world title," said Carl Moretti, vice president of Diaz promoter Top Rank. "He had really put in the work to get back to the level of being able to fight for a world title again."

Said Caldwell, "Juan is pretty bummed out but he's really optimistic. His thoughts are, 'Let's have the surgery and recover and get back on track.' He is definitely not giving up the dream of getting another world title.

"So he'll be out for about six months. It's too bad because the fight [with Ao] was a winnable fight. Tell Beltran and Ao that in six to eight months we're coming for that title."