British light heavyweight Joshua Buatsi secured a victory yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling single shots in a fight where the styles never quite clicked, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best performance but what defines champions? They prevail," Buatsi stated in the arena as he was faced boos from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers felt Parker, 31, deserved the decision but the British boxer endured the second loss of his 28-fight career.
The Champion Secures Victory in Uneventful Match
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who utilized a tight guard to narrowly win the cleaner rounds.
The audience's tolerance wore thin amid frequent holding, with Parker falling to the floor various instances during clinches.
Parker ended stronger, but several stanzas were hard to judge. However, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Homage Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a excellent all-Manchester fight to secure the European light heavyweight belt.
The stadium became quiet for a deeply poignant homage ahead of the contest as ten bells sounded in memory of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the last bell faded, the quiet transitioned to a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a left hook flush on the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in competitive blue - was marked up early, noticeable damage developing under his face as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur barely secured the decision with official scores of 114-114, narrow lead and wider gap, but both fighters were persistent and courageous, everything their hero Hatton stood for.
