British light heavyweight Joshua Buatsi made a winning yet disappointing debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful individual punches in a fight where the styles never quite clicked, with excessive holding disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi said in the arena as he was greeted with boos from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Various analysts believed Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
Buatsi Secures Victory in Slow Match
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He started carefully, with the two boxers ready to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who utilized a tight guard to barely take the better rounds.
The audience's tolerance diminished amid regular grabbing, with Parker going down various instances during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor moves on, but with the caliber of power puncher Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Arthur beat Bradley Rea by split-decision in a outstanding local rivalry contest to win the continental championship.
The arena grew hushed for a deeply poignant tribute before the fight as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the city since Hatton's death. As the final chime ended, the stillness gave way to a chant of "there's just one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was floored by a powerful punch flush on the chin in the initial stage, standing uncertainly to his position on the eight count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was marked up early, noticeable damage appearing under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor edged the verdict with judges scorecards of even score, 115-113 and clearer margin, but each competitor were determined and courageous, exactly what their inspiration the legend embodied.
