UK light heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques never quite clicked, with excessive holding spoiling the action.
The officials judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best showing but what do champions do? They succeed," the victor stated in the arena as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers felt Parker, 31, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Win in Uneventful Encounter
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to overcommit in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who worked behind a solid defense to edge the cleaner stanzas.
The audience's tolerance diminished because of constant clinching, with Parker going down several times during close exchanges.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Motivates Manchester Derby Battle
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding local rivalry fight to win the European light-heavyweight belt.
The arena grew hushed for a profoundly moving tribute prior to the bout as 10 bells tolled in remembrance of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the premier televised match card in the city since the passing of Hatton. As the final chime ended, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The action inside the ropes reflected the sentiment. The eventual winner was knocked down by a left hook squarely on the jaw in the second round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was marked up soon, significant bruising appearing under his face as the fighters exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the verdict with scoring tallies of draw, close margin and 115-112, but both fighters were determined and full of heart, all the qualities their idol Hatton represented.
