British light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But Parker found success, landing some telling individual punches in a fight where the techniques failed to mesh, with frequent clinching marring the fight.
The officials scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They prevail," the victor commented in the ringside as he was faced disapproval from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought Parker, 31, deserved the decision but the contender suffered the second defeat of his 28-fight professional journey.
The Champion Claims Win in Dull Affair
The fighter's advancement has stalled in the past few years through management issues and considerable time of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to achieve expectations - maybe in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors prepared to overcommit in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who operated with a protective stance to narrowly win the more effective rounds.
The audience's tolerance diminished during regular grabbing, with Parker going down several times during close exchanges.
The underdog closed well, but multiple periods were hard to judge. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of power puncher Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the co-main event, Arthur defeated Rea by divided decision in a excellent all-Manchester contest to win the continental title.
The stadium became quiet for a extremely emotional memorial prior to the bout as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away 46 years old in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the concluding sound ended, the silence gave way to a roar of "there's only one Hatton".
The fighting within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the face in the second round, rising shakily to his feet on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged quickly, heavy swelling forming under his orbital area as the fighters exchanged punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the verdict with judges scorecards of even score, 115-113 and clearer margin, but the two athletes were persistent and full of heart, all the qualities their idol Hatton embodied.
