British light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight ten-round fight at the boxing arena.
But Parker had his moments, delivering effective single shots in a fight where the techniques didn't complement each other, with constant grabbing spoiling the fight.
The officials had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They succeed," the victor commented in the arena as he was met by boos from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts believed Parker, 31, deserved the win but the contender experienced the second defeat of his 28-match professional journey.
Buatsi Earns Win in Slow Encounter
The fighter's advancement has plateaued in the past few years through contract problems and long spells of lack of fights.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter willing to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to barely take the cleaner rounds.
The spectators' acceptance diminished amid regular grabbing, with the challenger going down several times during grappling.
The underdog ended stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific all-Manchester battle to secure the European light heavyweight title.
The venue became quiet for a deeply poignant memorial prior to the bout as a bell ceremony sounded in honor of local hero Ricky Hatton, who died aged 46 in September.
It was the premier televised fight night in the location since Hatton's passing. As the last bell ended, the silence gave way to a chant of "there's only one Hatton".
The action during the bout matched the emotion. Arthur was knocked down by a left hook directly to the jaw in the initial stage, rising shakily to his position on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his orbital area as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor barely secured the result with official scores of draw, close margin and 115-112, but the two athletes were relentless and brave, everything their inspiration Hatton embodied.
