British light heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the approaches failed to mesh, with excessive holding spoiling the contest.
The scoring judges had it 96-94 for to the winner, with the final judge a even score.
"It wasn't my greatest performance but what do champions do? They prevail," the victor commented in the arena as he was met by disapproval from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th career victory.
Some ringside observers thought Parker, thirty-one, should have been awarded the victory but the contender experienced the second loss of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Match
The fighter's advancement has stalled in recent years through management issues and extended periods of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - possibly in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was making just his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective periods.
The crowd's patience wore thin because of frequent holding, with Parker falling to the floor several times during close exchanges.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur beat Rea by close verdict in a terrific all-Manchester battle to win the continental title.
The stadium fell silent for a deeply poignant memorial prior to the bout as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the first major televised boxing event in the location since Hatton's passing. As the concluding sound disappeared, the silence became a chant of "there's just one Ricky Hatton".
The action during the bout matched the emotion. The eventual winner was floored by a powerful punch flush on the face in the early round, standing uncertainly to his position on the referee's count.
But the red-colored attire of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was injured quickly, significant bruising appearing under his eye as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over the full distance.
Arthur edged the result with official scores of draw, 115-113 and clearer margin, but both fighters were relentless and courageous, exactly what their inspiration the legend represented.
