UK light heavyweight Joshua Buatsi earned a win yet disappointing start to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round bout at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a contest where the styles never quite clicked, with excessive holding spoiling the action.
The officials judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They succeed," the victor said in the arena as he was faced disapproval from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to earn a 20th career victory.
Various analysts felt Parker, thirty-one, should have been awarded the win but the contender endured the second defeat of his 28-fight professional journey.
Buatsi Earns Triumph in Slow Encounter
Buatsi's progress has stalled in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers prepared to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who worked behind a protective stance to barely take the more effective rounds.
The crowd's patience decreased because of frequent holding, with Parker falling to the floor multiple occasions during grappling.
Parker closed well, but multiple periods were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor progresses, but with the caliber of power puncher Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a terrific local rivalry fight to win the continental title.
The stadium grew hushed for a profoundly moving homage before the fight as 10 bells tolled in honor of Manchester's own Hatton, who passed away aged 46 in recently.
It was the first major televised fight night in the location since Hatton's passing. As the last bell disappeared, the stillness became a roar of "only one Ricky Hatton".
The competition during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook squarely on the jaw in the early round, rising shakily to his feet on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was marked up quickly, significant bruising forming under his orbital area as the pair landed hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with powerful upward punches as the intensity hardly diminished over twelve rounds.
The victor edged the decision with judges scorecards of even score, close margin and wider gap, but each competitor were persistent and full of heart, all the qualities their idol the legend stood for.
