British light-heavyweight Joshua Buatsi made a winning yet underwhelming debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close ten-round contest at the Manchester venue.
But the challenger landed significant shots, connecting with powerful single shots in a contest where the approaches failed to mesh, with frequent clinching disrupting the fight.
Two 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 win," Buatsi stated in the arena as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought Parker, 31, deserved the decision but the British boxer suffered the second setback of his 28-fight professional journey.
Buatsi Secures Victory in Dull Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand in the contest, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who operated with a solid defense to edge the more effective stanzas.
The crowd's patience wore thin during regular grabbing, with the challenger slipping to the canvas multiple occasions during grappling.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Fuels Local Rivalry Contest
In the co-main event, Arthur overcame Rea by close verdict in a outstanding local rivalry contest to win the European light-heavyweight belt.
The arena became quiet for a deeply poignant memorial before the fight as 10 bells sounded in memory of local hero Hatton, who succumbed 46 years old in September.
It was the premier televised boxing event in the city since Hatton's passing. As the final chime ended, the silence gave way to a shout of "only one Hatton".
The competition within the ring equaled the sentiment. Arthur was knocked down by a left hook directly to the face in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up soon, significant bruising forming under his face as the pair landed hard punches.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and full of heart, everything their inspiration Hatton represented.
