UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the techniques failed to mesh, with excessive holding spoiling the fight.
Two judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my finest performance but what defines champions? They prevail," the victor said in the ringside as he was met by negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, should have been awarded the victory but the British boxer suffered the second setback of his 28-fight career.
Buatsi Earns Triumph in Uneventful Encounter
His career development has stalled in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter prepared to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who utilized a solid defense to edge the cleaner stanzas.
The audience's tolerance diminished during regular grabbing, with Parker falling to the floor several times during grappling.
Parker closed well, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a terrific all-Manchester fight to secure the European light heavyweight title.
The arena fell silent for a extremely emotional homage before the fight as a bell ceremony rang in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the last bell ended, the silence transitioned to a roar of "there's just one Ricky Hatton".
The competition within the ring equaled the emotion. The eventual winner was floored by a powerful punch flush on the jaw in the initial stage, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, noticeable damage developing under his eye as the duo landed punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur narrowly won the result with scoring tallies of draw, 115-113 and wider gap, but both fighters were relentless and courageous, everything their inspiration the legend embodied.
