British light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a close ten-round contest at the boxing arena.
But the challenger found success, landing some telling clean blows in a contest where the styles didn't complement each other, with frequent clinching marring the action.
The officials judged it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my finest showing but what do champions do? They prevail," the victor said in the arena as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, should have been awarded the win but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
Buatsi Earns Victory in Slow Encounter
The fighter's advancement has slowed in recent years through contract problems and extended periods of lack of fights.
Previously predicted as the future of UK boxing, he now stands trying to realize his potential - perhaps in what feels like a last chance - under fresh management.
He started carefully, with both competitors ready to be aggressive in a cagey opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who utilized a solid defense to narrowly win the more effective periods.
The crowd's patience diminished during frequent holding, with the challenger going down several times during grappling.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester battle to win the continental championship.
The venue grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in last month.
It was the premier televised match card in the location since Hatton's death. As the last bell faded, the silence transitioned to a shout of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. The eventual winner was knocked down by a significant blow directly to the chin in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon advanced again. From the other corner, the opponent - in rival City blue - was damaged early, noticeable damage developing under his eye as the pair traded powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, fought fire with fire with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur edged the decision with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were relentless and full of heart, all the qualities their inspiration the legend embodied.
