British light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a tight 10-round fight at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a contest where the approaches didn't complement each other, with excessive holding disrupting the contest.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my finest performance but what defines champions? They win," Buatsi said in the ringside as he was greeted with boos from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, merited the win but the Derbyshire fighter experienced the second defeat of his 28-match professional journey.
The Champion Earns Triumph in Slow Match
Buatsi's progress has plateaued in recent times through management issues and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He started carefully, with the two boxers prepared to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who operated with a solid defense to barely take the more effective stanzas.
The spectators' acceptance decreased amid frequent holding, with Parker going down various instances during clinches.
The underdog ended stronger, but many rounds were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Battle
In the supporting bout, Arthur beat Rea by split-decision in a outstanding all-Manchester contest to capture the continental championship.
The stadium fell silent for a extremely emotional memorial before the fight as a bell ceremony rang in honor of the city's native Ricky Hatton, who passed away 46 years old in recently.
It was the initial significant broadcast match card in the city since Hatton's passing. As the last bell faded, the silence gave way to a roar of "only one Hatton".
The fighting within the ring equaled the feeling. The eventual winner was floored by a significant blow flush on the chin in the early round, getting up unsteadily to his feet on the referee's count.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was marked up early, noticeable damage developing under his orbital area as the fighters exchanged powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the intensity hardly diminished over 12 rounds.
The victor edged the decision with official scores of even score, 115-113 and 115-112, but both fighters were determined and brave, all the qualities their inspiration the legend embodied.
