UK light-heavyweight Joshua Buatsi made a winning yet unimpressive debut to his career under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a tight 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the approaches never quite clicked, with excessive holding disrupting the action.
The officials judged it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest display but what defines champions? They win," the victor commented in the ringside as he was met by negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the British boxer endured the second defeat of his 28-fight professional journey.
Buatsi Secures Triumph in Uneventful Match
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a final roll of the dice - under recent support.
He began cautiously, with neither fighter prepared to take risks in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who utilized a solid defense to narrowly win the more effective periods.
The audience's tolerance diminished during frequent holding, with Parker slipping to the canvas various instances during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, former world champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor progresses, but with the caliber of heavy hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Arthur defeated Rea by close verdict in a terrific local rivalry fight to capture the European light-heavyweight championship.
The arena fell silent for a profoundly moving tribute prior to the bout as a bell ceremony sounded in memory of the city's native Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the last bell faded, the silence became a shout of "there's only one Hatton".
The action inside the ropes reflected the feeling. Arthur was knocked down by a significant blow flush on the face in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was damaged early, noticeable damage forming under his face as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over 12 rounds.
The victor barely secured the decision with judges scorecards of 114-114, 115-113 and wider gap, but both fighters were determined and full of heart, all the qualities their hero Hatton stood for.
