UK light heavyweight Joshua Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a close ten-round fight at the boxing arena.
But the challenger found success, delivering effective clean blows in a fight where the styles didn't complement each other, with frequent clinching spoiling the fight.
The officials scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They win," Buatsi said in the ring as he was met by boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the win but the British boxer experienced the second defeat of his 28-fight career.
The Champion Secures Triumph in Dull Match
The fighter's advancement has plateaued in the past few years through contract problems and extended periods of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a last chance - under fresh management.
He started carefully, with neither fighter willing to be aggressive in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who utilized a solid defense to narrowly win the more effective stanzas.
The crowd's patience decreased amid constant clinching, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but many rounds were hard to judge. However, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the co-main event, Arthur overcame Bradley Rea by split-decision in a excellent local rivalry battle to secure the continental belt.
The arena fell silent for a deeply poignant homage before the fight as 10 bells tolled in honor of the city's native Hatton, who died at age 46 in recently.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime ended, the silence transitioned to a chant of "only one Hatton".
The action within the ring equaled the emotion. Arthur was floored by a powerful punch directly to the chin in the second round, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged quickly, heavy swelling appearing under his orbital area as the fighters traded powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the action seldom decreased over the full distance.
The victor edged the decision with judges scorecards of draw, narrow lead and wider gap, but each competitor were relentless and brave, exactly what their inspiration Hatton stood for.
