UK light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful clean blows in a fight where the techniques never quite clicked, with frequent clinching marring the contest.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but what do champions do? They prevail," Buatsi said in the arena as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, 31, merited the victory but the contender experienced the second setback of his 28-fight professional journey.
Buatsi Secures Win in Slow Encounter
The fighter's advancement has stalled in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a last chance - under fresh management.
He opened tentatively, with both competitors willing to overcommit in a calculated opening few rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to barely take the cleaner stanzas.
The audience's tolerance diminished amid constant clinching, with the challenger falling to the floor multiple occasions during grappling.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the co-main event, Arthur overcame Bradley Rea by divided decision in a terrific local rivalry battle to capture the continental belt.
The venue grew hushed for a profoundly moving tribute ahead of the contest as ten bells sounded in remembrance of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell ended, the silence gave way to a shout of "there's only one Ricky Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a significant blow directly to the face in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was marked up soon, significant bruising developing under his eye as the pair exchanged hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the verdict with official scores of draw, narrow lead and wider gap, but each competitor were persistent and courageous, all the qualities their idol the legend embodied.
