UK light-heavyweight Joshua Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a close 10-round bout at the boxing arena.
But the challenger landed significant shots, connecting with powerful individual punches in a match where the techniques didn't complement each other, with excessive holding marring the fight.
The scoring judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They win," the victor stated in the arena as he was faced boos from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts thought the challenger, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
Buatsi Secures Win in Slow Encounter
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to realize his potential - possibly in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to take risks in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a tight guard to barely take the more effective periods.
The audience's tolerance decreased because of regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. However, ex-champion Carl Frampton - providing analysis for the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific all-Manchester contest to secure the European light heavyweight title.
The arena grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died at age 46 in last month.
It was the premier televised fight night in the location since Hatton's death. As the concluding sound disappeared, the silence transitioned to a shout of "there's only one Ricky Hatton".
The fighting inside the ropes reflected the feeling. Arthur was knocked down by a left hook flush on the face in the second round, rising shakily to his position on the referee's count.
But the red-colored attire of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was marked up soon, heavy swelling developing under his orbital area as the duo exchanged hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with official scores of even score, close margin and wider gap, but the two athletes were determined and brave, everything their inspiration the legend embodied.
