British light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a match where the techniques didn't complement each other, with constant grabbing marring the fight.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They prevail," the victor stated in the arena as he was greeted with disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt Parker, 31, deserved the decision but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Secures Win in Dull Match
His career development has slowed in the past few years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a solid defense to edge the cleaner rounds.
The spectators' acceptance decreased amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the co-main event, Arthur overcame Rea by close verdict in a excellent Manchester vs Manchester battle to capture the European light heavyweight championship.
The venue grew hushed for a extremely emotional memorial before the fight as 10 bells rang in memory of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the concluding sound ended, the silence transitioned to a roar of "there's just one Hatton".
The action within the ring matched the feeling. Arthur was sent to the canvas by a powerful punch flush on the face in the early round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was injured quickly, significant bruising developing under his face as the fighters landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the result with official scores of even score, 115-113 and clearer margin, but each competitor were persistent and brave, all the qualities their idol Hatton embodied.
