British light-heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a close 10-round bout at the boxing arena.
But the challenger found success, delivering effective clean blows in a contest where the approaches didn't complement each other, with constant grabbing marring the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but what do champions do? They prevail," the victor commented in the arena as he was met by disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Some ringside observers believed Parker, 31, merited the win but the contender suffered the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Dull Encounter
The fighter's advancement has stalled in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with neither fighter willing to be aggressive in a calculated early stages.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was making just his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who utilized a solid defense to edge the more effective stanzas.
The audience's tolerance decreased amid constant clinching, with the challenger falling to the floor several times during grappling.
The underdog closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Arthur beat Bradley Rea by close verdict in a excellent local rivalry contest to secure the continental belt.
The stadium became quiet for a extremely emotional tribute prior to the bout as a bell ceremony tolled in remembrance of the city's native Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's death. As the last bell disappeared, the stillness became a chant of "there's only one Hatton".
The action inside the ropes matched the sentiment. Arthur was knocked down by a significant blow squarely on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was injured early, heavy swelling appearing under his eye as the pair landed punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the decision with judges scorecards of draw, narrow lead and wider gap, but the two athletes were persistent and full of heart, all the qualities their hero the legend stood for.
