UK light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a fight where the styles failed to mesh, with excessive holding spoiling the fight.
The officials judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Several boxing experts thought the challenger, 31, should have been awarded the win but the Derbyshire fighter experienced the second defeat of his 28-bout career.
Buatsi Secures Triumph in Uneventful Affair
The fighter's advancement has plateaued in recent years through promotional disputes and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter ready to be aggressive in a tentative early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who worked behind a solid defense to edge the more effective stanzas.
The audience's tolerance diminished amid regular grabbing, with the challenger falling to the floor various instances during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - based on this performance - still appears challenging.
Hatton Homage Motivates All-Manchester Fight
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a terrific all-Manchester battle to win the continental title.
The arena became quiet for a profoundly moving memorial prior to the bout as a bell ceremony rang in remembrance of local hero Ricky Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound faded, the silence became a shout of "only one Ricky Hatton".
The fighting within the ring reflected the emotion. The eventual winner was knocked down by a significant blow squarely on the jaw in the initial stage, rising shakily to his position on the eight count.
But the traditional red attire of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was injured early, heavy swelling forming under his orbital area as the pair landed hard punches.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the pace hardly diminished over the full distance.
The victor barely secured the decision with judges scorecards of even score, close margin and clearer margin, but both fighters were relentless and brave, all the qualities their idol the legend represented.
