UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a match where the techniques never quite clicked, with frequent clinching marring the contest.
The officials judged it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They win," the victor stated in the ring as he was met by negative reactions from the crowd.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Various analysts believed Parker, 31, deserved the victory but the contender experienced the second setback of his 28-fight career.
The Champion Secures Victory in Uneventful Match
His career development has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under new promotional backing.
He began cautiously, with both competitors willing to take risks in a calculated initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who utilized a solid defense to narrowly win the better rounds.
The audience's tolerance diminished because of frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog closed well, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Hatton Memorial Inspires Manchester Derby Battle
In the supporting bout, Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to win the European light heavyweight championship.
The arena grew hushed for a extremely emotional homage prior to the bout as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed aged 46 in September.
It was the initial significant broadcast match card in the location since Hatton's passing. As the concluding sound faded, the silence gave way to a roar of "only one Hatton".
The fighting inside the ropes equaled the emotion. Arthur was knocked down by a powerful punch flush on the face in the second round, getting up unsteadily to his position on the referee's count.
But the traditional red shorts of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was injured quickly, heavy swelling developing under his face as the fighters landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor edged the verdict with official scores of 114-114, 115-113 and wider gap, but each competitor were determined and brave, all the qualities their hero the legend embodied.
