UK light heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close 10-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques failed to mesh, with excessive holding spoiling the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They win," the victor stated in the ringside as he was met by boos from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the win but the British boxer experienced the second setback of his 28-bout career.
Buatsi Claims Win in Slow Encounter
Buatsi's progress has slowed in recent times through contract problems and extended periods of inactivity.
Once tipped as the next star of British boxing, he now stands trying to achieve expectations - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who utilized a protective stance to barely take the cleaner stanzas.
The crowd's patience decreased because of constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor progresses, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the co-main event, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester fight to win the European light heavyweight title.
The stadium became quiet for a extremely emotional memorial prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the final chime ended, the silence became a chant of "there's only one Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a powerful punch directly to the face in the initial stage, rising shakily to his position on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured early, significant bruising developing under his face as the duo landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the action rarely dropped over 12 rounds.
Arthur barely secured the result with official scores of draw, close margin and 115-112, but each competitor were persistent and courageous, everything their inspiration the legend stood for.
