British light heavyweight Joshua Buatsi made a winning yet disappointing start to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the styles never quite clicked, with constant grabbing disrupting the contest.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the ring as he was met by negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the British boxer suffered the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Dull Match
His career development has stalled in recent years through contract problems and considerable time of lack of fights.
Earlier forecast as the future of UK boxing, he now finds himself trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to edge the cleaner periods.
The audience's tolerance decreased amid frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. However, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor advances, but with the presence of hard hitter Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester contest to win the European light heavyweight belt.
The stadium grew hushed for a profoundly moving homage ahead of the contest as a bell ceremony sounded in honor of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the location since Hatton's death. As the final chime faded, the silence gave way to a shout of "there's only one Hatton".
The action within the ring reflected the emotion. The eventual winner was knocked down by a significant blow directly to the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up soon, noticeable damage forming under his face as the pair exchanged powerful shots.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor barely secured the verdict with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were persistent and courageous, exactly what their idol the legend embodied.
