UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a fight where the approaches didn't complement each other, with excessive holding spoiling the fight.
The officials scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my best performance but what defines champions? They win," the victor stated in the ringside as he was met by boos from the spectators.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers felt Parker, thirty-one, should have been awarded the win but the contender experienced the second setback of his 28-fight career.
Buatsi Earns Triumph in Slow Match
Buatsi's progress has plateaued in recent years through contract problems and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now finds himself trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to edge the better stanzas.
The audience's tolerance wore thin amid constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog closed well, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to win the European light heavyweight championship.
The arena grew hushed for a profoundly moving tribute before the fight as a bell ceremony tolled in memory of the city's native Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound faded, the quiet transitioned to a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was floored by a powerful punch squarely on the face in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was damaged early, noticeable damage appearing under his face as the fighters landed powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
The victor edged the decision with judges scorecards of 114-114, close margin and wider gap, but both fighters were relentless and brave, all the qualities their hero Hatton stood for.
