British light heavyweight Joshua Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive ten-round fight at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a match where the styles never quite clicked, with constant grabbing disrupting the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best showing but what defines champions? They win," the victor said in the ring as he was faced disapproval from the spectators.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers thought Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
Buatsi Earns Victory in Uneventful Match
His career development has slowed in the past few years through management issues and extended periods of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He started carefully, with neither fighter willing to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to edge the better periods.
The audience's tolerance decreased amid constant clinching, with the challenger going down multiple occasions during clinches.
Parker closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Arthur beat Bradley Rea by divided decision in a excellent all-Manchester fight to secure the continental belt.
The venue became quiet for a deeply poignant memorial before the fight as ten bells tolled in memory of local hero Ricky Hatton, who succumbed at age 46 in September.
It was the first major televised match card in the city since Hatton's passing. As the last bell disappeared, the stillness transitioned to a roar of "there's only one Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was floored by a powerful punch directly to the chin in the second round, rising shakily to his feet on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was marked up early, noticeable damage developing under his orbital area as the pair traded punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the pace hardly diminished over the full distance.
The victor edged the verdict with judges scorecards of draw, 115-113 and 115-112, but each competitor were persistent and full of heart, exactly what their idol the legend stood for.
