UK light-heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling individual punches in a match where the techniques failed to mesh, with excessive holding marring the fight.
Two judges scored it 96-94 to Buatsi, with the final judge a 95-95.
"It wasn't my best performance but what defines champions? They prevail," the victor commented in the ring as he was met by disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Various analysts thought Parker, 31, should have been awarded the victory but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Earns Win in Slow Affair
Buatsi's progress has slowed in the past few years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to achieve expectations - maybe in what appears to be a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a tight guard to barely take the more effective stanzas.
The audience's tolerance decreased because of frequent holding, with Parker going down several times during close exchanges.
The underdog ended stronger, but multiple periods were difficult to score. However, ex-champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the co-main event, Arthur overcame Bradley Rea by divided decision in a terrific all-Manchester battle to win the continental championship.
The stadium fell silent for a deeply poignant tribute before the fight as 10 bells rang in memory of the city's native Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast fight night in the urban center since the passing of Hatton. As the last bell disappeared, the quiet gave way to a shout of "there's just one Ricky Hatton".
The action during the bout matched the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the chin in the second round, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was damaged early, significant bruising developing under his eye as the fighters traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
The victor narrowly won the verdict with official scores of even score, narrow lead and clearer margin, but both fighters were relentless and brave, all the qualities their idol Hatton stood for.
