After three rounds of sparring on Saturday, John (42-0-2, 22 KOs) had a fainting spell, said Golden Boy Promotions CEO Richard Schaefer.

"He is out. He has something with the blood that diminishes his capacity as an athlete," Lewkowicz said. "I don't know exactly what it is but he is feeling ill. He went to the doctor and he had a check up and he had a blood test, and something is wrong. I don't know exactly what it is but the doctor said he can't train or exercise. He was training, but was feeling fatigued and went to the doctor."

Lewkowicz said John planned to see his doctor again on Monday for additional tests.

"He will stay in the States until we get a full and comprehensive treatment," Lewkowicz said. "It's a blow for all of us. I hope he will feel better, but he won't be fighting."

Lewkowicz said John took the blood test required by the California commission before a fight two weeks ago and it came back fine.

"This is nothing like hepatitis or HIV or anything," Lewkowicz said. "It's something like anemia, like he's missing oxygen in the blood. When he did his blood test two weeks ago, he was fine. He took this one because he felt ill and they found out something was wrong."

Schaefer said he notified HBO executives of the situation on Sunday and that they would discuss options on Monday.

"I just know Chris John is out and they are looking at some possible replacements," Schaefer said, referring to Golden Boy matchmakers Eric Gomez and Robert Diaz. "Our goal is to keep Rocky on the show and have him fight."

Schaefer said a potential opponent is Mexico's Juan Carlos Salgado (20-0, 14 KOs), who is regarded as a quality prospect. Even if they can sign Salgado for the fight, HBO would still have to approve of him as an opponent.

Juarez manager Shelly Finkel, who arrived home from Germany after his client Wladimir Klitschko's heavyweight championship victory against Ruslan Chagaev on Saturday, said Juarez was disappointed, but they were hopeful he would remain on the card.

"We're hoping Rocky stays on the card," Finkel said. "He is ready to fight."

John, 29, has defended his title 11 times. Juarez, 29, a 2000 U.S. Olympic silver medalist, has come up short in five shots at various titles at featherweight and junior lightweight.

Junior welterweight Victor Ortiz, the 2008 ESPN.com prospect of the year, will face Argentina's Marco Maidana for a vacant interim belt in the main event.