British light heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a close ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a contest where the approaches failed to mesh, with constant grabbing marring the fight.
Two judges had it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi stated in the ring as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Several boxing experts believed Parker, 31, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-match professional journey.
Buatsi Secures Victory in Uneventful Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors willing to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to edge the cleaner stanzas.
The crowd's patience diminished amid frequent holding, with the challenger going down several times during grappling.
Parker finished the stronger, but multiple periods were difficult to score. However, former world champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester contest to secure the continental title.
The venue grew hushed for a profoundly moving tribute prior to the bout as 10 bells tolled in memory of local hero Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the city since Hatton's death. As the concluding sound disappeared, the silence gave way to a shout of "only one Ricky Hatton".
The competition during the bout reflected the feeling. Arthur was sent to the canvas by a significant blow flush on the chin in the initial stage, rising shakily to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured quickly, significant bruising appearing under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over the full distance.
Arthur narrowly won the result with judges scorecards of even score, narrow lead and clearer margin, but each competitor were determined and brave, exactly what their idol the legend stood for.
