British light heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a match where the approaches never quite clicked, with frequent clinching marring the action.
The officials judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," Buatsi commented in the arena as he was greeted with disapproval from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer endured the second defeat of his 28-bout career.
Buatsi Earns Triumph in Dull Affair
The fighter's advancement has slowed in recent times through management issues and considerable time of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a last chance - under fresh management.
He opened tentatively, with both competitors prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for the favorite, who utilized a solid defense to edge the more effective periods.
The spectators' acceptance diminished during regular grabbing, with Parker falling to the floor several times during clinches.
Parker closed well, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry fight to win the European light heavyweight belt.
The stadium fell silent for a extremely emotional homage ahead of the contest as a bell ceremony tolled in memory of Manchester's own Hatton, who passed away at age 46 in last month.
It was the premier televised match card in the city since Hatton's death. As the concluding sound faded, the stillness transitioned to a shout of "there's only one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a left hook flush on the jaw in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was damaged early, significant bruising appearing under his face as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity seldom decreased over the full distance.
The victor barely secured the result with official scores of 114-114, close margin and wider gap, but the two athletes were relentless and courageous, everything their hero Hatton embodied.
