UK light-heavyweight Joshua Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close 10-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a match where the techniques never quite clicked, with frequent clinching disrupting the fight.
Two judges had it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They prevail," the victor stated in the ringside as he was faced disapproval from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-bout professional journey.
Buatsi Secures Win in Slow Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to barely take the cleaner stanzas.
The audience's tolerance decreased during frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker ended stronger, but many rounds were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the continental title.
The arena grew hushed for a profoundly moving homage ahead of the contest as 10 bells sounded in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the final chime faded, the quiet transitioned to a chant of "there's only one Ricky Hatton".
The fighting during the bout reflected the feeling. Arthur was knocked down by a left hook flush on the jaw in the initial stage, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of the fighter soon advanced again. Across the ring, Rea - in rival City blue - was injured soon, heavy swelling developing under his face as the fighters exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over the full distance.
The victor edged the result with official scores of draw, 115-113 and clearer margin, but each competitor were relentless and courageous, exactly what their idol the legend stood for.
