UK light-heavyweight Joshua Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the techniques didn't complement each other, with frequent clinching spoiling the action.
Two judges had it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest showing but what do champions do? They prevail," the victor stated in the ringside as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts felt Parker, thirty-one, should have been awarded the win but the British boxer suffered the second defeat of his 28-bout professional journey.
The Champion Secures Win in Slow Affair
Buatsi's progress has slowed in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to achieve expectations - possibly in what seems like a final roll of the dice - under recent support.
He opened tentatively, with both competitors ready to take risks in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who operated with a protective stance to barely take the better periods.
The spectators' acceptance decreased amid constant clinching, with the challenger slipping to the canvas various instances during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry battle to win the European light-heavyweight title.
The venue grew hushed for a deeply poignant tribute ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who died at age 46 in recently.
It was the premier televised match card in the location since Hatton's death. As the final chime faded, the silence gave way to a chant of "there's only one Ricky Hatton".
The fighting within the ring matched the feeling. Arthur was knocked down by a significant blow directly to the chin in the initial stage, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was marked up soon, heavy swelling developing under his face as the duo landed hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur narrowly won the decision with official scores of 114-114, close margin and wider gap, but the two athletes were relentless and brave, everything their idol the legend embodied.
