British light-heavyweight Joshua Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a fight where the techniques failed to mesh, with frequent clinching marring the contest.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest performance but how do champions respond? They win," the victor said in the arena as he was greeted with disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Various analysts felt Parker, 31, deserved the win but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
The Champion Secures Victory in Uneventful Affair
His career development has stalled in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under fresh management.
He started carefully, with the two boxers prepared to take risks in a tentative opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for Buatsi, who worked behind a solid defense to barely take the better rounds.
The crowd's patience diminished because of constant clinching, with the challenger slipping to the canvas several times during close exchanges.
The underdog closed well, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to secure the European light-heavyweight championship.
The stadium became quiet for a profoundly moving tribute ahead of the contest as ten bells rang in memory of local hero Hatton, who died aged 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime ended, the silence transitioned to a roar of "there's only one Ricky Hatton".
The action within the ring equaled the feeling. Arthur was floored by a left hook directly to the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was marked up early, noticeable damage developing under his face as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over 12 rounds.
The victor edged the verdict with official scores of even score, close margin and 115-112, but each competitor were persistent and courageous, all the qualities their inspiration the legend stood for.
