UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a majority decision in a tight 10-round contest at the Manchester venue.
But Parker found success, connecting with powerful clean blows in a fight where the approaches failed to mesh, with frequent clinching spoiling the contest.
The officials scored it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest showing but what defines champions? They win," the victor commented in the ringside as he was faced negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, merited the decision but the contender suffered the second loss of his 28-match career.
Buatsi Secures Victory in Uneventful Match
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the future of the British fight scene, he now is positioned trying to achieve expectations - maybe in what seems like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers ready to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for the favorite, who worked behind a tight guard to edge the better periods.
The crowd's patience wore thin because of constant clinching, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but several stanzas were difficult to score. However, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific local rivalry fight to secure the continental title.
The venue became quiet for a profoundly moving tribute before the fight as a bell ceremony rang in honor of Manchester's own Hatton, who passed away at age 46 in recently.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound faded, the quiet became a shout of "only one Ricky Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was sent to the canvas by a left hook directly to the face in the early round, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, Rea - in opposing blue - was injured early, significant bruising developing under his orbital area as the fighters landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur narrowly won the verdict with official scores of even score, narrow lead and wider gap, but each competitor were persistent and courageous, everything their idol the legend stood for.
