Robert Guerrero has shoulder surgery

Former two-division titlist Robert Guerrero underwent complication-free surgery to repair the partially torn rotator cuff in his left shoulder on Tuesday and is expected to be able to resume full training in about four months, manager Bob Santos told ESPN.com.

During an Aug. 17 sparring session, Guerrero suffered the shoulder injury, which forced him to pull out of a much-anticipated HBO main event against junior welterweight titlist Marcos Maidana scheduled for Aug. 27 at the HP Pavilion in San Jose, Calif., near Guerrero's hometown of Gilroy.

It was a bad blow for Guerrero, a former featherweight and junior lightweight titlist, who was on the verge of major fights and about to participate in his first HBO main event.

Santos said the arthroscopic surgery, which lasted about two hours, was performed by Dr. Michael Dillingham, the San Francisco 49ers team physician, in Burlingame, Calif. Guerrero (29-1-1, 18 KOs), 28, was already resting at home Tuesday evening.

"Dr. Dillingham is a very humble doctor, so he is not one of these kind of guys who will jump up and down and say, 'Hey, it's a success!' But in his opinion, everything was OK and we are very optimistic," Santos said. "He felt he was able to fix the tear."

During Dillingham's initial exam, he told Guerrero he felt that the injury probably stemmed from an injury he had suffered as a youth football running back when he was 13 or 14. The shoulder has given Guerrero problems on and off for years.

"The doctor looked at the rotator cuff and said he probably hurt it when he was playing football, that this was not just something that happened in one sparring session," Santos said. "He said the whole thing has been damaged for years, but that he went ahead and fixed everything. He was amazed that Robert has been fighting his whole career with this. He said the rotator cuff was all scar tissue and had to have been there for years. The reason he could deal with it throughout his career was because of his pain threshold. But once it tore more when he was sparring, it was a different feeling.

"The doctor was very optimistic about things. Robert has been dealing with it for years and never knew it, but went about his business and sucked it up. But whatever the doctor could do to his best surgical ability, that is what he did."

Guerrero will be in a sling for about two weeks before he can begin rehabilitation. Santos said the doctor said Guerrero would probably be able to punch with his left hand in about four months.

"Robert was relieved to go in and get the surgery done," Santos said. "God-willing, everything will OK. He's just 28 and there is no reason to rush him."

Dan Rafael is the boxing writer for ESPN.com. Follow him on Twitter @danrafaelespn.