British light heavyweight Joshua Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close ten-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful single shots in a contest where the techniques failed to mesh, with frequent clinching disrupting the fight.
Two judges had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but what do champions do? They succeed," Buatsi said in the ring as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt Parker, 31, merited the win but the British boxer suffered the second defeat of his 28-match career.
Buatsi Secures Triumph in Dull Affair
Buatsi's progress has plateaued in recent years through contract problems and considerable time of inactivity.
Previously predicted as the future of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers willing to overcommit in a cagey initial rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to narrowly win the more effective periods.
The spectators' acceptance decreased because of regular grabbing, with Parker slipping to the canvas various instances during clinches.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by divided decision in a excellent all-Manchester contest to win the European light heavyweight title.
The stadium fell silent for a extremely emotional memorial ahead of the contest as 10 bells rang in honor of the city's native Ricky Hatton, who died 46 years old in last month.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell faded, the quiet became a roar of "there's only one Ricky Hatton".
The fighting within the ring reflected the emotion. Arthur was knocked down by a powerful punch squarely on the face in the second round, rising shakily to his footing on the referee's count.
But the traditional red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged early, heavy swelling developing under his face as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the decision with judges scorecards of draw, narrow lead and 115-112, but the two athletes were relentless and full of heart, everything their inspiration Hatton embodied.
