Heavyweight titleholder Ruslan Chagaev suffered a complete tear of the Achilles tendon in his left ankle Thursday, forcing him to withdraw from a July 5 mandatory defense in Germany against former beltholder Nikolai Valuev.
Chagaev was in the penultimate round on his final day of sparring when he suffered the injury, promoter Universum Box-Promotion announced.
"He made a step back towards the left and immediately I heard something. From that moment I knew something was wrong," trainer Michael Timm said.
"It felt like as a stone hit my Achilles," Chagaev said. "I can only say that this is the biggest upset in my career. But I will come back."
Chagaev, who was immediately taken to a hospital in Hamburg, is scheduled for surgery on Monday.
"The injury isn't as bad as it sounds," said Dr. Michael Ehnert. "The diagnoses is actually something positive because the tendon has (torn) in the middle and not by the heel bone. Therefore, we hope for a faster recovery."
The doctor estimated that Chagaev would be ready to fight again before the end of the year.
It is the third time since winning a majority decision against Valuev, 34, to claim the WBA belt in April 2007 that Chagaev has pulled out of a fight due to injury or illness.
Chagaev and the 7-foot, 320-pound Valuev were supposed to have their rematch on May 31, but Chagaev pulled out because of illness.
Chagaev (24-0-1, 17 KOs), an Uzbekistan native based in Germany, was supposed to face then-titleholder Sultan Ibragimov in a unification fight last fall, but withdrew because of an undisclosed medical issue described by Universum at the time as "an acute inflammation of the body." Numerous sources reported that he was suffering from hepatitis B.
Chagaev, 29, has made one defense, a lopsided points win against England's Matt Skelton on Jan. 19.
Don King, who co-promotes Russia's Valuev (48-1, 34 KOs) with Sauerland Event, was disgusted with Chagaev.
"How many times is this dude going to pull out of a fight," King said. "He doesn't really want to fight the giant (Valuev) again. Valuev has trained for this fight twice. I want to see if the WBA will allow him to fight for the interim title immediately."
Universum's July 5 card will still go on with middleweight titleholder Felix Sturm's rematch with American Randy Griffin, with whom he drew last fall, moving from the undercard into the main event position.
Dan Rafael is the boxing writer for ESPN.com.