UK light heavyweight Joshua Buatsi made a winning yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a tight 10-round bout at the boxing arena.
But the challenger found success, landing some telling single shots in a fight where the approaches failed to mesh, with excessive holding disrupting the action.
The officials judged it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They succeed," the victor commented in the arena as he was met by negative reactions from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
Buatsi Secures Triumph in Uneventful Encounter
His career development has plateaued in recent years through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter ready to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a tight guard to narrowly win the more effective stanzas.
The spectators' acceptance decreased during regular grabbing, with Parker going down several times during grappling.
The underdog ended stronger, but multiple periods were hard to judge. However, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Inspires Manchester Derby Fight
In the co-main event, Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester contest to capture the continental championship.
The stadium grew hushed for a extremely emotional homage before the fight as ten bells rang in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound ended, the silence gave way to a chant of "there's just one Hatton".
The action within the ring matched the feeling. Arthur was knocked down by a significant blow directly to the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up early, heavy swelling appearing under his eye as the pair landed hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur narrowly won the decision with official scores of draw, 115-113 and wider gap, but the two athletes were determined and full of heart, everything their inspiration the legend stood for.
