Bob Arum admits he underestimated Teofimo Lopez.

The 23-year-old lightweight champion’s promoter knew Lopez was a powerful puncher. Arum – whose company, Top Rank Inc., represents Lopez and Lomachenko – just didn’t realize that the confident, entertaining Lopez is capable of boxing as well as he did versus Vasiliy Lomachenko in what amounted to a career-changing performance Saturday night in Las Vegas.

Brooklyn’s Lopez (16-0, 12 KOs) built a large lead on the scorecards and out-boxed the skillful southpaw during their 12-round, 135-pound title unification fight at MGM Grand Conference Center. Lopez upset Ukraine’s Lomachenko (14-2, 10 KOs) by unanimous decision to become the youngest fully unified champion in boxing’s four-belt era.

“The kid surprised me in the sense that I knew he was good, but I didn’t realize how good,” Arum told BoxingScene.com. “Because we’ve never seen a performance by him like this. We always saw him go out and whack some poor unfortunate guy out, and that’s all we thought he had. But obviously, he has a lot more.”

His father/trainer, Teofimo Lopez Sr., assured detractors that his son was much more than just the knockout artist who demolished Richard Commey to win the IBF lightweight title and previously tore through other lesser opponents. Lopez stopped Ghana’s Commey (29-3, 26 KOs) in the second round December 14 at Madison Square Garden in New York, but odds-makers still installed Lomachenko as a 4-1 favorite to beat him in a main event ESPN televised.

Lopez’s hand speed and power made Lomachenko cautious during the first half of their fight. Lomachenko came back to an extent during the second half of their bout, but judges Julie Lederman (119-109), Steve Weisfeld (117-111) and Tim Cheatham (116-112) still scored 11, nine and eight rounds, respectively, for Lopez.

The underdog retained his IBF lightweight title, and won the WBA “super,” WBC “franchise” and WBO belts from Lomachenko. He also ended Lomachenko’s 13-fight winning streak by becoming the first fighter in 6½ years to beat one of the top pound-for-pound boxers in the sport.

“He fought like a real mature guy,” Arum said. “You’ve gotta give his father credit, because the father really trains him. That was a magnificent job that he did, having him fight the fight that he did. It was like a seasoned professional. A really seasoned old-timer would fight a fight like that.”

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