Hector Garcia capitalized on the opportunity of his lifetime Saturday night.

The largely unknown underdog overwhelmed heavily favored 130-pound contender Chris Colbert en route to beating up the brash Brooklyn native and winning a 12-round unanimous decision in Showtime’s main event from The Chelsea inside The Cosmopolitan of Las Vegas. Colbert (16-1, 6 KOs) never dealt well with the Dominican southpaw’s pressure or body punching.

Judges Dave Moretti (118-109), Patricia Morse Jarman (119-108) and Steve Weisfeld (118-109) all gave Garcia clear credit for his work, which included a seventh-round knockdown.

Garcia’s confidence never wavered, as he hammered Colbert to the head and body in a completely one-sided fight. Caesars Sportsbook listed Colbert as a 22-1 favorite before Garcia, a late replacement for WBA champion Roger Gutierrez, upset him.

The 30-year-old Garcia (15-0, 10 KOs, 3 NC), a 2016 Olympian for the Dominican Republic, had more than 300 amateur fights. His experience was as evident as his toughness during just his second televised fight in the United States.

“When I was told three weeks ago that I was gonna take this fight, I saw in my eyes that my life was gonna change,” Garcia told Showtime’s Jim Gray through a translator. “So, I needed to come in my best shape.”

Garcia produced a career-best performance without his head trainer, Ismael Salas, in his corner. Assistant trainer Bob Santos served as Garcia’s chief second because Salas traveled to Glasgow, Scotland, for two-time Olympic gold medalist Robeisy Ramirez’s third-round stoppage of Eric Donovan on the Josh Taylor-Jack Catterall undercard at OVO Hydro.

“I wanna give a lot of credit to the professor, Ismael Salas,” Garcia added, “who gave me the confidence to say, ‘You can win this fight.’ ”

Garcia took this fight on less than three weeks’ notice. Colbert was supposed to battle Gutierrez for Gutierrez’s WBA world super featherweight title, but Gutierrez contracted COVID-19 while training and withdrew from what was supposed to be a mandatory title defense.

Colbert-Garcia was an elimination match, thus Garcia expects to challenge Argentina’s Gutierrez (26-3-1, 20 KOs) in his next fight.

Colbert graciously gave Garcia credit for his accomplishment.

“I’d like to thank everybody that came out here to support me,” Colbert said. “I’m sorry I let y’all down, but tonight he was just a better man. I’m gonna take my defeats as I take my [wins], as a man. I’m gonna keep my head up and I wanna run it back. Let’s do it again.”

Colbert wouldn’t use a late switch in opponents, from a right-handed fighter to a southpaw, as justification for suffering his first defeat.

“No, no, no. It’s no letdown, it’s no none of that,” Colbert said. “It’s no excuses. I’m gonna take my losses the way I take my wins. He was the better man tonight. Let’s run it back.”

In need of a knockout, Colbert tried to bang it out with Garcia as soon as the 12th round began. Garcia’s power quickly made him think twice about that strategy and forced Colbert to stay away from him.

Obviously ahead on points, Garcia battered Colbert throughout a one-sided 11th round. Colbert moved away from him at times, when he clearly needed to attack an opponent who obviously was well ahead on points.

Referee Kenny Bayless admonished Colbert for shoving Garcia 35 seconds into the 10th round. A straight left by Garcia backed Colbert into a corner with 1:35 to go in the 10th round.

Colbert held Garcia with just over a minute to go in the 10th round. Garcia later unloaded a combination on Colbert when he was backed into a corner.

Colbert blasted Garcia with a flush right hand during the eighth round, but Garcia took it well. That at least provided evidence that Colbert would fight hard to remain unbeaten.

After sending Colbert to the canvas during the seventh round, Garcia overpowered him again during the eighth round. Garcia unloaded power punches to the head and body in the eighth round, when he backed a game but out-gunned Colbert into the ropes and beat him up.

Garcia stung Colbert with two jabs that connected with just over 1:40 remaining in the seventh round. A counter left hand by Garcia knocked Colbert flat on his back with 1:15 on the clock in the seventh round.

Colbert got up from the first knockdown of his career by the time Bayless counted to five. He held once the action resumed and moved his way toward the bell to end the round, yet not before Garcia landed a couple more hard lefts.

A counter right hand by Colbert landed early in the sixth round. Colbert backed into a corner later in the sixth round and waved Garcia forward.

Garcia obliged by landing hard shots to Colbert’s head and body. He stopped Colbert in his tracks with a left hand that landed with 30 seconds to go in the sixth round.

Garcia tattooed Colbert with a straight left up top that backed him into the ropes with 25 seconds on the clock in the fifth round. The underdog had his way with Colbert in that round when Colbert backed into the ropes, though Colbert succeeded early in the fifth when he kept the action in the middle of the ring.

An aggressive Garcia pressured Colbert throughout the fourth round, when he kept throwing body shots and consistently moved Colbert backward. Colbert waved him forward, but clearly had difficulty dealing with Garcia’s body work and activity.

Colbert again stood his ground and slugged it out with Garcia in the third round. Garcia missed many of his attempts to Colbert’s head, yet he kept catching Colbert to his body.

Colbert drilled Garcia with a right to his body with about 35 seconds to go in the third round. Garcia landed a straight left in the final 10 seconds of the third round, which made Colbert fire back with a right hand.

Colbert stood and traded with Garcia throughout the second round, but Garcia got the better of the action again in those three minutes. Garcia continued to go after Colbert’s body and caught Colbert with a counter right hook toward the end of the second round.

Garcia landed a hard left to Colbert’s body and then caught him with a stiff jab in the opening minute of the first round. Garcia continued to hit Colbert with lefts to his body during the first round.

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