Mike Tyson was dismissive when a Tyson Fury-Francis Ngannou fight was first mentioned to him.

The legendary former heavyweight champion considered it a preposterous proposition because Fury has proven himself as the best heavyweight of this generation and Ngannou has never participated in a professional boxing match. Tyson changed his mind when he saw Ngannou spar for the first time in Las Vegas.

Once Tyson watched the former UFC heavyweight champion injure a sparring partner who is as tall as Fury, Tyson began to believe that Ngannou would have a legitimate chance to upset the unbeaten WBC heavyweight champ. Tyson trained Ngannou along with Dewey Cooper for his 10-round, non-title fight against Fury on Saturday night at Kingdom Arena in Riyadh, Saudi Arabia.

Tyson, 57, acknowledged during their final press conference Thursday night in Riyadh that he was very skeptical of this fight once upon a time.

“Hey, listen, I thought this was ridiculous, too, until I saw him spar,” Tyson said. “You know, I watched him box around and he moved. But when I saw him hit this guy – this guy is 6-9, 6-8, big, strong. When he hit the guy, the guy, I never saw nothing like that before. … He hit the guy on the chin and he broke the ankle [or] the leg. Hey, listen, man, if he could do that it’s possible he can be heavyweight champ of the world. It’s been a long time since I seen a knockout [like that]. It really wasn’t a knockout, it was a leg break or something.”

The 37-year-old Ngannou, of Batie, Cameroon, is commonly considered one of the hardest punchers in UFC history. The 35-year-old Fury, of Morecambe, England, has violently knocked out one of the most pulverizing punchers in boxing history in Deontay Wilder, but Tyson feels that the 6-foot-4, 272-pound Ngannou poses a legitimate threat to Fury because of his unusual power.

“Oh, it’s definitely real,” Tyson said. “It’s definitely real.”

The 6-foot-9, 277¾-pound Fury is nevertheless a 20-1 favorite to beat the hard-hitting novice, according to FanDuel sportsbook.

Ngannou, who is 17-3 (12 KOs) in MMA matches, will make his professional boxing debut. He has lost only on points in MMA bouts.

Fury-Ngannou will top a TNT Sports Box Office pay-per-view event in the United Kingdom (£21.95; 7 p.m. BST). ESPN will distribute Fury-Ngannou as the main event of a pay-per-view show in the United States ($79.99; 2 p.m. EDT; 11 a.m. PDT).

Keith Idec is a senior writer/columnist for BoxingScene.com. He can be reached on Twitter @Idecboxing.