British light heavyweight Joshua Buatsi earned a win yet disappointing beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a contest where the styles never quite clicked, with frequent clinching marring the fight.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi stated in the ringside as he was met by boos from the crowd.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to earn a 20th career victory.
Several boxing experts felt Parker, 31, should have been awarded the victory but the contender endured the second loss of his 28-match professional journey.
The Champion Earns Triumph in Dull Encounter
Buatsi's progress has stalled in recent times through contract problems and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent 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 made it competitive, making life awkward for the favorite, who utilized a protective stance to barely take the better rounds.
The crowd's patience wore thin because of frequent holding, with the challenger going down several times during close exchanges.
The underdog closed well, but several stanzas were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Arthur overcame Rea by close verdict in a excellent all-Manchester contest to secure the European light-heavyweight championship.
The stadium fell silent for a deeply poignant memorial before the fight as a bell ceremony tolled in remembrance of Manchester's own Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound ended, the quiet transitioned to a roar of "only one Hatton".
The action during the bout reflected the sentiment. The eventual winner was sent to the canvas by a left hook flush on the chin in the initial stage, getting up unsteadily to his position on the count of eight.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was injured early, noticeable damage appearing under his orbital area as the fighters exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur edged the result with scoring tallies of draw, close margin and wider gap, but each competitor were relentless and courageous, all the qualities their inspiration the legend embodied.
