British light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a match where the approaches never quite clicked, with frequent clinching disrupting the contest.
The officials had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They prevail," the victor commented in the ring as he was met by boos from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Several boxing experts believed Parker, thirty-one, merited the victory but the contender endured the second loss of his 28-bout professional journey.
Buatsi Claims Win in Slow Affair
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of lack of fights.
Once tipped as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who operated with a protective stance to narrowly win the more effective rounds.
The crowd's patience decreased during frequent holding, with Parker falling to the floor multiple occasions during clinches.
The underdog closed well, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a outstanding all-Manchester battle to capture the European light heavyweight title.
The venue became quiet for a deeply poignant memorial ahead of the contest as ten bells sounded in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime ended, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The action during the bout matched the emotion. Arthur was sent to the canvas by a significant blow flush on the chin in the early round, rising shakily to his position on the count of eight.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured quickly, noticeable damage forming under his face as the pair exchanged punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
Arthur edged the verdict with official scores of draw, close margin and wider gap, but both fighters were determined and brave, all the qualities their inspiration Hatton stood for.
