British light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a close 10-round bout at the Manchester venue.
But Parker found success, delivering effective individual punches in a match where the approaches didn't complement each other, with frequent clinching disrupting the action.
The scoring judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor said in the arena as he was faced disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers thought Parker, thirty-one, should have been awarded the decision but the British boxer endured the second setback of his 28-fight career.
Buatsi Secures Victory in Slow Encounter
Buatsi's progress has plateaued in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the future of the British fight scene, he now finds himself trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter ready to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who worked behind a solid defense to edge the better rounds.
The audience's tolerance diminished amid frequent holding, with Parker going down multiple occasions during clinches.
The underdog closed well, but many rounds were difficult to score. However, former world champion the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester fight to secure the continental championship.
The arena fell silent for a profoundly moving tribute ahead of the contest as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away at age 46 in last month.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell ended, the quiet gave way to a shout of "there's just one Hatton".
The action during the bout reflected the sentiment. The eventual winner was sent to the canvas by a powerful punch directly to the face in the early round, standing uncertainly to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up soon, heavy swelling forming under his eye as the duo traded punishing blows.
Rea, 27, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor narrowly won the verdict with judges scorecards of even score, 115-113 and clearer margin, but both fighters were persistent and brave, exactly what their hero the legend represented.
