UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a match where the styles didn't complement each other, with constant grabbing marring the action.
The officials judged it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best display but how do champions respond? They win," the victor stated in the ring as he was faced disapproval from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, should have been awarded the victory but the contender suffered the second loss of his 28-fight professional journey.
Buatsi Claims Victory in Dull Encounter
Buatsi's progress has slowed in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to realize his potential - maybe in what seems like a final roll of the dice - under fresh management.
He started carefully, with both competitors ready to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who operated with a tight guard to narrowly win the cleaner stanzas.
The crowd's patience decreased during frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. Although, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Inspires Manchester Derby Fight
In the supporting bout, Arthur overcame Rea by split-decision in a terrific all-Manchester contest to secure the European light-heavyweight title.
The stadium grew hushed for a deeply poignant memorial before the fight as 10 bells rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the concluding sound ended, the quiet became a roar of "only one Ricky Hatton".
The competition during the bout matched the emotion. Arthur was sent to the canvas by a left hook directly to the chin in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured soon, heavy swelling developing under his face as the pair landed punishing blows.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the result with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and full of heart, all the qualities their inspiration the legend embodied.
