British light heavyweight Joshua Buatsi secured a victory yet underwhelming start to his career under new promoters 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 10-round contest at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a contest where the approaches didn't complement each other, with constant grabbing disrupting the contest.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my best showing but what defines champions? They prevail," Buatsi said in the arena as he was greeted with boos from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers believed Parker, thirty-one, deserved the win but the British boxer endured the second defeat of his 28-fight career.
The Champion Claims Victory in Slow Encounter
The fighter's advancement has stalled in recent years through management issues and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter ready to take risks in a cagey initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who utilized a solid defense to edge the more effective stanzas.
The audience's tolerance wore thin because of constant clinching, with the challenger going down multiple occasions during grappling.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a outstanding all-Manchester fight to secure the European light-heavyweight title.
The arena grew hushed for a profoundly moving tribute prior to the bout as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died at age 46 in recently.
It was the first major televised match card in the urban center since the passing of Hatton. As the concluding sound faded, the silence gave way to a roar of "only one Hatton".
The competition during the bout equaled the sentiment. The eventual winner was floored by a left hook flush on the face in the initial stage, rising shakily to his feet on the count of eight.
But the traditional red attire of the fighter soon came forward again. From the other corner, the opponent - in competitive blue - was damaged quickly, significant bruising developing under his face as the pair exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the verdict with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were relentless and full of heart, all the qualities their idol Hatton embodied.
