UK light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the styles never quite clicked, with excessive holding spoiling the fight.
Two judges scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the arena as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the decision but the British boxer suffered the second defeat of his 28-match career.
The Champion Earns Victory in Slow Match
Buatsi's progress has stalled in recent years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - possibly 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 calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who utilized a tight guard to barely take the better stanzas.
The spectators' acceptance diminished because of frequent holding, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry battle to capture the European light heavyweight title.
The stadium fell silent for a deeply poignant memorial prior to the bout as a bell ceremony tolled in memory of the city's native Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the final chime ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The action during the bout reflected the emotion. The eventual winner was knocked down by a significant blow squarely on the chin in the initial stage, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up soon, heavy swelling developing under his face as the duo exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the verdict with official scores of 114-114, close margin and clearer margin, but both fighters were relentless and courageous, exactly what their inspiration Hatton represented.
