UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger had his moments, landing some telling clean blows in a match where the techniques didn't complement each other, with constant grabbing disrupting the fight.
Two judges scored it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," the victor commented in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the victory but the contender suffered the second setback of his 28-bout career.
The Champion Secures Triumph in Slow Encounter
Buatsi's progress has slowed in the past few years through contract problems and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a tentative opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who utilized a solid defense to narrowly win the cleaner periods.
The audience's tolerance diminished because of constant clinching, with Parker going down various instances during close exchanges.
Parker ended stronger, but multiple periods were challenging to assess. However, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light heavyweight title.
The stadium fell silent for a extremely emotional tribute prior to the bout as a bell ceremony rang in memory of local hero Hatton, who succumbed 46 years old in September.
It was the first major televised match card in the city since Hatton's death. As the last bell faded, the silence transitioned to a chant of "there's only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was knocked down by a powerful punch squarely on the jaw in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up soon, noticeable damage appearing under his orbital area as the fighters landed hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over the full distance.
The victor narrowly won the result with judges scorecards of even score, 115-113 and wider gap, but both fighters were determined and courageous, exactly what their hero the legend embodied.
