UK light-heavyweight Joshua Buatsi secured a victory yet disappointing beginning to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker had his moments, landing some telling individual punches in a match where the approaches never quite clicked, with frequent clinching spoiling the fight.
Two judges judged it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They prevail," Buatsi said in the ring as he was met by boos from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the victory but the contender experienced the second defeat of his 28-bout professional journey.
Buatsi Earns Victory in Uneventful Encounter
The fighter's advancement has stalled in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after breaking his hand mid-fight, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a tight guard to barely take the better periods.
The crowd's patience decreased during frequent holding, with Parker going down several times during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Arthur defeated Rea by divided decision in a terrific Manchester vs Manchester battle to capture the continental title.
The arena grew hushed for a deeply poignant homage prior to the bout as 10 bells tolled in honor of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the premier televised fight night in the urban center since Hatton's death. As the final chime faded, the quiet became a chant of "only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was floored by a left hook flush on the chin in the initial stage, rising shakily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling appearing under his face as the duo exchanged punishing blows.
Rea, 27, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur barely secured the result with scoring tallies of 114-114, close margin and wider gap, but the two athletes were persistent and full of heart, all the qualities their idol the legend stood for.
