UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a close ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective single shots in a fight where the styles never quite clicked, with constant grabbing marring the contest.
Two judges scored it 96-94 in favor to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but what defines champions? They succeed," the victor commented in the arena as he was met by negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed the challenger, 31, merited the victory but the Derbyshire fighter experienced the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Dull Match
Buatsi's progress has slowed in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what feels like a last chance - under recent support.
He opened tentatively, with both competitors prepared to take risks in a calculated early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who utilized a solid defense to barely take the cleaner rounds.
The spectators' acceptance wore thin amid regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Tribute Inspires All-Manchester Fight
In the co-main event, Arthur beat Rea by split-decision in a terrific all-Manchester battle to win the European light-heavyweight belt.
The venue grew hushed for a profoundly moving memorial ahead of the contest as 10 bells sounded in memory of Manchester's own Hatton, who passed away at age 46 in September.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound ended, the quiet transitioned to a shout of "there's only one Ricky Hatton".
The fighting during the bout equaled the feeling. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, standing uncertainly to his position on the referee's count.
But the traditional red attire of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured quickly, heavy swelling forming under his orbital area as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the verdict with official scores of draw, close margin and clearer margin, but each competitor were relentless and full of heart, exactly what their hero Hatton embodied.
