UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a majority decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a fight where the techniques failed to mesh, with constant grabbing disrupting the fight.
The officials had it 96-94 for to the winner, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They win," the victor stated in the ringside as he was greeted with disapproval from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the win but the British boxer suffered the second loss of his 28-fight career.
Buatsi Secures Triumph in Slow Encounter
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers ready to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who operated with a tight guard to edge the better periods.
The spectators' acceptance diminished during constant clinching, with the challenger falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Hatton Homage Inspires All-Manchester Battle
In the co-main event, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester contest to win the continental title.
The venue became quiet for a profoundly moving homage ahead of the contest as 10 bells sounded in honor of the city's native Ricky Hatton, who passed away 46 years old in last month.
It was the premier televised boxing event in the urban center since the passing of Hatton. As the concluding sound faded, the quiet transitioned to a shout of "there's only one Hatton".
The fighting during the bout reflected the emotion. Arthur was knocked down by a left hook directly to the jaw in the second round, rising shakily to his position on the count of eight.
But the traditional red equipment of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was damaged quickly, heavy swelling appearing under his eye as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur edged the result with official scores of even score, narrow lead and 115-112, but each competitor were persistent and full of heart, all the qualities their inspiration the legend stood for.
