British light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close 10-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful single shots in a match where the techniques didn't complement each other, with excessive holding marring the action.
The officials judged it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," the victor said in the ring as he was met by boos from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, deserved the win but the contender suffered the second setback of his 28-match career.
Buatsi Claims Triumph in Dull Match
The fighter's advancement has stalled in recent times through management issues and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with neither fighter prepared to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a tight guard to narrowly win the better periods.
The crowd's patience wore thin amid frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker finished the stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester battle to win the European light heavyweight championship.
The arena became quiet for a deeply poignant memorial prior to the bout as a bell ceremony tolled in honor of Manchester's own Hatton, who died at age 46 in last month.
It was the premier televised match card in the location since Hatton's death. As the concluding sound faded, the silence transitioned to a roar of "only one Ricky Hatton".
The action during the bout equaled the sentiment. Arthur was sent to the canvas by a significant blow flush on the face in the initial stage, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was injured early, significant bruising developing under his face as the pair exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur narrowly won the result with judges scorecards of draw, 115-113 and clearer margin, but both fighters were persistent and brave, everything their inspiration the legend stood for.
