An unidentified blood ailment has forced featherweight titleholder Chris John to withdraw from Saturday's rematch with Rocky Juarez, according to ESPN.com's Dan Rafael.

John (42-0-2, 22 KOs) reportedly fainted after three rounds of sparring Saturday. A blood test revealed “something with the blood that diminishes his capacity as an athlete” John's adviser Sampson Lewkowicz told Rafael Sunday night.

Lewkowicz added that the problem was nothing life threatening or permanent such as the HIV or hepatitis B or C viruses.

It sounds like common anemia, but the illness makes John, THE RING's No. 1-rated featherweight contender who held Juarez to an entertaining draw (that many observers thought he won) in February, the latest in an ever-growing list of top fighters who have had to pull out of major fights scheduled this summer.

The John-Juarez rematch was to be the HBO-televised co-feature to the Victor-Ortiz Marcos Maidana showdown that will headline at STAPLES Center in Los Angeles this Saturday.

Juarez will be kept on the card, Golden Boy Promotions matchmaker Eric Gomez told Rafael. A replacement for Juarez is being sought.