UK light-heavyweight Joshua Buatsi made a winning yet disappointing debut 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 judges' verdict in a tight 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a fight where the styles never quite clicked, with excessive holding spoiling the contest.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest performance but what do champions do? They win," Buatsi stated in the ring as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, deserved the victory but the contender suffered the second defeat of his 28-fight professional journey.
Buatsi Claims Triumph in Slow Affair
Buatsi's progress has stalled in the past few years through management issues and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with neither fighter ready to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who utilized a solid defense to barely take the more effective stanzas.
The crowd's patience wore thin because of constant clinching, with the challenger slipping to the canvas various instances during grappling.
The underdog closed well, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a excellent local rivalry fight to win the European light heavyweight championship.
The arena grew hushed for a deeply poignant tribute ahead of the contest as ten bells rang in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound ended, the quiet became a chant of "there's just one Hatton".
The competition inside the ropes equaled the feeling. Arthur was floored by a powerful punch directly to the face in the second round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was marked up soon, significant bruising developing under his eye as the duo landed punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the decision with judges scorecards of draw, 115-113 and clearer margin, but both fighters were persistent and brave, everything their hero the legend stood for.
