UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the styles never quite clicked, with frequent clinching spoiling the contest.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," the victor said in the arena as he was greeted with disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts thought Parker, 31, should have been awarded the win but the contender suffered the second defeat of his 28-fight professional journey.
Buatsi Secures Triumph in Dull Affair
Buatsi's progress has stalled in recent years through contract problems and long spells of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a final roll of the dice - under recent support.
He began cautiously, with neither fighter willing to overcommit in a tentative early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was participating in his second fight at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to barely take the cleaner stanzas.
The audience's tolerance diminished amid frequent holding, with Parker going down multiple occasions during grappling.
Parker closed well, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
The victor advances, but with the caliber of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the continental championship.
The venue became quiet for a deeply poignant tribute ahead of the contest as a bell ceremony sounded in honor of local hero Hatton, who died at age 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound disappeared, the stillness became a chant of "there's only one Hatton".
The action within the ring equaled the feeling. Arthur was sent to the canvas by a significant blow squarely on the chin in the initial stage, rising shakily to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged quickly, heavy swelling developing under his orbital area as the pair exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the verdict with official scores of even score, narrow lead and wider gap, but both fighters were persistent and full of heart, exactly what their idol Hatton represented.
