UK light heavyweight Joshua Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective single shots in a fight where the approaches never quite clicked, with constant grabbing disrupting the fight.
The scoring judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They succeed," the victor stated in the arena as he was greeted with boos from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second defeat of his 28-fight career.
Buatsi Claims Triumph in Uneventful Encounter
His career development has slowed in recent times through promotional disputes and long spells of inactivity.
Once tipped as the future of UK boxing, he now stands trying to realize his potential - possibly in what appears to be a last chance - under fresh management.
He started carefully, with the two boxers ready to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to edge the cleaner stanzas.
The crowd's patience diminished amid constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were hard to judge. Although, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Homage Inspires All-Manchester Contest
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester contest to win the European light heavyweight title.
The arena grew hushed for a extremely emotional memorial ahead of the contest as 10 bells tolled in memory of local hero Hatton, who died at age 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the last bell disappeared, the silence transitioned to a shout of "there's just one Hatton".
The action inside the ropes reflected the sentiment. Arthur was knocked down by a significant blow directly to the jaw in the initial stage, rising shakily to his position on the count of eight.
But the red-colored attire of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged early, noticeable damage appearing under his face as the pair landed powerful shots.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace seldom decreased over the full distance.
The victor edged the result with official scores of even score, narrow lead and clearer margin, but each competitor were persistent and courageous, everything their hero the legend stood for.
