British light-heavyweight Joshua Buatsi made a winning yet unimpressive beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight ten-round fight at the Manchester venue.
But the challenger found success, delivering effective single shots in a fight where the approaches didn't complement each other, with excessive holding spoiling the contest.
The scoring judges scored it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They prevail," the victor said in the ringside as he was greeted with disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Various analysts thought Parker, thirty-one, merited the victory but the contender suffered the second loss of his 28-bout professional journey.
Buatsi Claims Triumph in Dull Match
Buatsi's progress has plateaued in the past few years through contract problems and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors ready to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a protective stance to edge the more effective rounds.
The audience's tolerance decreased because of constant clinching, with the challenger falling to the floor multiple occasions during clinches.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Hatton Homage Fuels All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester contest to win the European light heavyweight belt.
The stadium became quiet for a profoundly moving homage ahead of the contest as ten bells rang in honor of local hero Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the final chime disappeared, the stillness became a shout of "there's just one Hatton".
The action during the bout reflected the sentiment. The eventual winner was knocked down by a significant blow directly to the chin in the early round, getting up unsteadily to his position on the count of eight.
But the traditional red attire of Arthur soon advanced again. On the opposite side, the opponent - in rival City blue - was damaged soon, significant bruising forming under his eye as the pair traded powerful shots.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor narrowly won the verdict with official scores of draw, narrow lead and 115-112, but each competitor were persistent and brave, exactly what their idol Hatton stood for.
