UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a match where the approaches never quite clicked, with excessive holding spoiling the contest.
The scoring judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what defines champions? They win," the victor said in the arena as he was met by boos from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th pro success.
Several boxing experts believed Parker, thirty-one, deserved the win but the contender experienced the second setback of his 28-match professional journey.
Buatsi Earns Win in Slow Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what feels like a last chance - under fresh management.
He started carefully, with both competitors prepared to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who worked behind a solid defense to barely take the more effective periods.
The spectators' acceptance diminished amid constant clinching, with the challenger falling to the floor multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. However, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding local rivalry contest to win the continental title.
The venue grew hushed for a profoundly moving memorial ahead of the contest as 10 bells sounded in memory of the city's native Ricky Hatton, who passed away aged 46 in September.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound faded, the silence transitioned to a roar of "there's just one Hatton".
The competition inside the ropes equaled the emotion. Arthur was sent to the canvas by a significant blow flush on the face in the early round, getting up unsteadily to his feet on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured quickly, significant bruising developing under his eye as the duo traded hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor edged the result with official scores of draw, 115-113 and clearer margin, but each competitor were relentless and courageous, all the qualities their hero Hatton embodied.
