British light heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful clean blows in a match where the techniques failed to mesh, with excessive holding disrupting the fight.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor said in the arena as he was greeted with disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, merited the decision but the contender experienced the second defeat of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Affair
The fighter's advancement has stalled in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter ready to take risks in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the cleaner stanzas.
The audience's tolerance diminished during regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog ended stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of power puncher David Benavidez, and Russian star Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a outstanding all-Manchester battle to win the European light heavyweight title.
The stadium fell silent for a profoundly moving homage prior to the bout as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died 46 years old in recently.
It was the first major televised boxing event in the location since Hatton's death. As the last bell faded, the quiet transitioned to a roar of "there's just one Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was sent to the canvas by a left hook flush on the face in the initial stage, rising shakily to his feet on the eight count.
But the traditional red shorts of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was damaged early, significant bruising appearing under his orbital area as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor edged the decision with scoring tallies of even score, 115-113 and clearer margin, but both fighters were determined and brave, exactly what their idol the legend represented.
