British light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a majority decision in a tight 10-round contest at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a match where the styles didn't complement each other, with frequent clinching spoiling the fight.
The officials judged it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They succeed," Buatsi said in the ring as he was faced negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
Buatsi Secures Win in Slow Affair
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of inactivity.
Earlier forecast as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter ready to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand in the contest, was participating in his second appearance at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to edge the more effective periods.
The spectators' acceptance wore thin amid frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a terrific all-Manchester battle to secure the European light heavyweight belt.
The stadium fell silent for a deeply poignant homage before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the urban center since Hatton's passing. As the last bell faded, the silence gave way to a chant of "there's only one Hatton".
The fighting during the bout reflected the feeling. The eventual winner was knocked down by a significant blow flush on the chin in the second round, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up soon, heavy swelling appearing under his eye as the duo traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the result with judges scorecards of 114-114, narrow lead and wider gap, but both fighters were persistent and courageous, exactly what their inspiration Hatton stood for.
