UK light heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a tight ten-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a match where the techniques never quite clicked, with excessive holding marring the contest.
The scoring judges scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They prevail," the victor stated in the arena as he was met by disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Some ringside observers felt Parker, 31, merited the win but the Derbyshire fighter experienced the second defeat of his 28-bout career.
Buatsi Secures Triumph in Uneventful Affair
The fighter's advancement has slowed in recent years through promotional disputes and long spells of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He opened tentatively, with neither fighter ready to take risks in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the more effective rounds.
The spectators' acceptance decreased during frequent holding, with Parker going down multiple occasions during clinches.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific all-Manchester battle to win the continental championship.
The arena grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony rang in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised match card in the urban center since Hatton's death. As the final chime ended, the silence transitioned to a roar of "only one Ricky Hatton".
The competition within the ring reflected the sentiment. The eventual winner was knocked down by a significant blow squarely on the jaw in the second round, rising shakily to his footing on the count of eight.
But the red-colored shorts of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was injured early, significant bruising appearing under his eye as the duo exchanged hard punches.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, 115-113 and wider gap, but the two athletes were persistent and courageous, all the qualities their inspiration the legend represented.
