Sergey "Krusher" Kovalev is set to defend his WBA, IBF, and WBO titles on Saturday vs. Nadjib Mohammedi and he is a massive favorite. Kovalev is one of the premier punchers in the sport-as evidenced by his 86 percent knockout rate-and he proved he has a ton of skill too in a unanimous decision victory over Bernard Hopkins in November 2014.

So how could an unbeaten light heavyweight gotten knocked around by Gennady Golovkin, a middleweight, in sparring?

That is a tale that has widely circulated thanks to Golovkin's trainer Abel Sanchez, who is filling in the gaps ahead of Kovalev's clash. Sanchez used to train both men, and explained that Kovalev was a raw, inexperienced fighter when he put him in to spar with Golovkin.

"When Kovalev first came to me he had nine fights. He was a young man, not in age but in boxing ability. When he came to me Golovkin was already a world champion, and a silver medalist in the Olympics, over 300 amateur fights...he had had all these credentials.

"So when they sparred it was like a very experienced guy against a very inexperienced guy, so of course Golovkin got the best of him," Sanchez said.

Sanchez credited Kovalev's new trainer, John David Jackson with turning him into a killer. "That guy we see on Saturday is not the guy who left my gym, because the guy who left my gym was still a pup.

"So as far as him and Golovkin sparring, yes Golovkin got the better of him. But it wasn't just Kovalev who got bested-it was everyone in the gym that sparred Golovkin-he was able to maneuver them and do what he wanted to do."

[Boxing Scene]