British light-heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight ten-round fight at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the approaches never quite clicked, with constant grabbing marring the action.
The officials judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor said in the arena as he was met by boos from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts felt Parker, 31, merited the win but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Earns Victory in Dull Encounter
The fighter's advancement has stalled in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a last chance - under recent support.
He began cautiously, with both competitors ready to take risks in a calculated opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a tight guard to edge the more effective periods.
The audience's tolerance wore thin amid frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog closed well, but multiple periods were challenging to assess. However, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Homage Motivates Local Rivalry Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a excellent all-Manchester fight to win the European light heavyweight title.
The venue became quiet for a extremely emotional memorial prior to the bout as ten bells tolled in memory of Manchester's own Ricky Hatton, who died 46 years old in recently.
It was the premier televised boxing event in the location since the passing of Hatton. As the last bell ended, the silence gave way to a chant of "there's only one Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was knocked down by a left hook directly to the face in the early round, getting up unsteadily to his position on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was damaged soon, heavy swelling developing under his eye as the fighters landed punishing blows.
The challenger, 27, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur narrowly won the verdict with official scores of even score, close margin and 115-112, but the two athletes were relentless and brave, all the qualities their inspiration Hatton stood for.
