UK light heavyweight Joshua Buatsi earned a win yet disappointing start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a majority decision in a tight 10-round bout at the boxing arena.
But the challenger found success, delivering effective clean blows in a match where the techniques never quite clicked, with constant grabbing disrupting the fight.
Two judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They win," Buatsi commented in the arena as he was met by disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the decision but the British boxer experienced the second defeat of his 28-match professional journey.
Buatsi Claims Win in Uneventful Encounter
Buatsi's progress has slowed in recent years through management issues and long spells of lack of fights.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter ready to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to barely take the better rounds.
The crowd's patience decreased because of frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester contest to capture the continental championship.
The venue grew hushed for a profoundly moving memorial before the fight as ten bells sounded in remembrance of the city's native Hatton, who passed away aged 46 in September.
It was the premier televised match card in the city since Hatton's death. As the concluding sound disappeared, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The competition within the ring equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the face in the initial stage, getting up unsteadily to his position on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up quickly, heavy swelling developing under his eye as the pair traded powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace hardly diminished over 12 rounds.
Arthur edged the decision with official scores of even score, close margin and 115-112, but both fighters were relentless and brave, all the qualities their hero the legend stood for.
