UK light heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a close 10-round bout at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a fight where the approaches didn't complement each other, with excessive holding marring the action.
The officials had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi stated in the ringside as he was faced boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Various analysts felt Parker, 31, merited the victory but the British boxer experienced the second defeat of his 28-fight career.
The Champion Secures Triumph in Slow Affair
Buatsi's progress has slowed in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the next star of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for Buatsi, who worked behind a tight guard to edge the better periods.
The spectators' acceptance wore thin during regular grabbing, with Parker falling to the floor multiple occasions during grappling.
Parker closed well, but several stanzas were hard to judge. However, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Homage Inspires All-Manchester Battle
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to capture the European light-heavyweight title.
The venue fell silent for a deeply poignant tribute before the fight as ten bells tolled in memory of local hero Hatton, who died aged 46 in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the last bell disappeared, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The competition inside the ropes matched the emotion. The eventual winner was knocked down by a powerful punch flush on the face in the early round, rising shakily to his position on the referee's count.
But the red-colored attire of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was marked up early, significant bruising forming under his eye as the pair landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur edged the result with official scores of 114-114, close margin and clearer margin, but each competitor were persistent and full of heart, all the qualities their inspiration Hatton embodied.
