UK light-heavyweight Joshua Buatsi secured a victory yet unimpressive debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a tight 10-round contest at the Co-op Live Arena.
But Parker found success, landing some telling clean blows in a fight where the approaches failed to mesh, with excessive holding marring the action.
The officials had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," the victor stated in the ring as he was faced disapproval from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th career victory.
Several boxing experts thought Parker, 31, deserved the victory but the contender endured the second loss of his 28-bout professional journey.
The Champion Secures Victory in Uneventful Encounter
The fighter's advancement has slowed in the past few years through management issues and extended periods of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - maybe in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with the two boxers willing to overcommit in a tentative opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for the favorite, who worked behind a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance diminished during frequent holding, with Parker going down several times during clinches.
Parker finished the stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Tribute Motivates Local Rivalry Battle
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to win the European light heavyweight championship.
The stadium grew hushed for a profoundly moving memorial before the fight as ten bells tolled in honor of local hero Hatton, who passed away 46 years old in recently.
It was the first major televised boxing event in the urban center since Hatton's death. As the concluding sound faded, the quiet transitioned to a chant of "only one Hatton".
The action inside the ropes matched the sentiment. The eventual winner was knocked down by a powerful punch directly to the face in the second round, rising shakily to his footing on the referee's count.
But the red-colored shorts of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was injured soon, heavy swelling forming under his face as the duo landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur barely secured the result with official scores of even score, narrow lead and clearer margin, but the two athletes were persistent and courageous, all the qualities their idol Hatton represented.
