UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close ten-round contest at the Manchester venue.
But the challenger had his moments, delivering effective individual punches in a contest where the styles never quite clicked, with excessive holding marring the action.
The officials scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor commented in the ring as he was met by boos from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the win but the contender endured the second defeat of his 28-bout professional journey.
Buatsi Claims Victory in Uneventful Affair
His career development has slowed in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a last chance - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to edge the cleaner rounds.
The crowd's patience wore thin during constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog closed well, but many rounds were difficult to score. However, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a outstanding all-Manchester fight to capture the European light heavyweight championship.
The stadium grew hushed for a deeply poignant memorial prior to the bout as 10 bells sounded in memory of the city's native Ricky Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the last bell disappeared, the stillness gave way to a roar of "there's only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a powerful punch flush on the face in the early round, rising shakily to his footing on the eight count.
But the traditional red attire of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was damaged soon, noticeable damage forming under his eye as the fighters exchanged punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur narrowly won the verdict with official scores of draw, 115-113 and 115-112, but each competitor were determined and courageous, everything their hero the legend embodied.
