British light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive 10-round contest at the Manchester venue.
But the challenger found success, connecting with powerful clean blows in a fight where the approaches never quite clicked, with frequent clinching spoiling the contest.
Two judges scored it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They win," the victor commented in the ringside as he was met by negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers felt the challenger, 31, should have been awarded the victory but the Derbyshire fighter experienced the second loss of his 28-fight professional journey.
The Champion Claims Triumph in Dull Encounter
His career development has stalled in recent times through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors ready to overcommit in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was making just his second fight at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who operated with a protective stance to narrowly win the better rounds.
The audience's tolerance wore thin amid constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur beat Rea by divided decision in a outstanding local rivalry contest to secure the European light heavyweight belt.
The arena grew hushed for a profoundly moving tribute ahead of the contest as ten bells tolled in honor of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the premier televised match card in the location since Hatton's death. As the last bell ended, the stillness transitioned to a roar of "there's only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, heavy swelling developing under his orbital area as the pair landed hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity seldom decreased over the full distance.
Arthur narrowly won the decision with judges scorecards of draw, 115-113 and wider gap, but both fighters were persistent and courageous, everything their inspiration the legend represented.
