UK light-heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the techniques never quite clicked, with constant grabbing disrupting the contest.
The scoring judges judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best display but what defines champions? They prevail," Buatsi said in the ring as he was met by disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers felt Parker, thirty-one, deserved the decision but the British boxer suffered the second loss of his 28-fight professional journey.
Buatsi Earns Triumph in Uneventful Encounter
His career development has stalled in recent times through management issues and considerable time of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with the two boxers prepared to take risks in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a protective stance to narrowly win the more effective stanzas.
The audience's tolerance decreased during frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
Parker closed well, but many rounds were difficult to score. However, former world champion the boxing analyst - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Hatton Homage Inspires Local Rivalry Fight
In the co-main event, Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the European light heavyweight championship.
The venue grew hushed for a profoundly moving memorial before the fight as 10 bells sounded in honor of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the premier televised fight night in the location since the passing of Hatton. As the final chime faded, the silence became a roar of "there's only one Hatton".
The fighting inside the ropes reflected the sentiment. Arthur was knocked down by a left hook squarely on the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged quickly, heavy swelling developing under his orbital area as the pair landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over the full distance.
The victor edged the decision with judges scorecards of even score, 115-113 and clearer margin, but both fighters were persistent and brave, exactly what their hero Hatton embodied.
