UK light heavyweight Joshua Buatsi made a winning yet underwhelming start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a close 10-round fight at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the techniques never quite clicked, with excessive holding disrupting the action.
Two judges had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what do champions do? They win," Buatsi said in the ringside as he was greeted with boos from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the victory but the contender suffered the second setback of his 28-fight career.
Buatsi Secures Win in Uneventful Affair
Buatsi's progress has plateaued in recent times through management issues and long spells of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He started carefully, with both competitors willing to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was participating in his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for the favorite, who utilized a tight guard to narrowly win the cleaner stanzas.
The crowd's patience decreased amid constant clinching, with Parker going down various instances during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Fuels Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a outstanding all-Manchester fight to capture the European light heavyweight championship.
The stadium became quiet for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in honor of the city's native Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the final chime ended, the stillness became a shout of "there's just one Ricky Hatton".
The action during the bout matched the feeling. Arthur was sent to the canvas by a left hook flush on the jaw in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was marked up soon, heavy swelling developing under his face as the duo landed punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the intensity rarely dropped over 12 rounds.
The victor barely secured the verdict with judges scorecards of draw, narrow lead and wider gap, but each competitor were relentless and brave, everything their inspiration the legend embodied.
