UK light-heavyweight Joshua Buatsi made a winning yet underwhelming start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a fight where the styles didn't complement each other, with frequent clinching disrupting the action.
The officials had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but what do champions do? They prevail," the victor said in the ringside as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Several boxing experts believed the challenger, thirty-one, merited the victory but the contender endured the second setback of his 28-fight career.
Buatsi Secures Triumph in Uneventful Match
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under fresh management.
He started carefully, with both competitors willing to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to barely take the better periods.
The crowd's patience wore thin amid frequent holding, with the challenger falling to the floor multiple occasions during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Inspires All-Manchester Fight
In the supporting bout, Arthur defeated Rea by close verdict in a excellent all-Manchester battle to secure the European light-heavyweight championship.
The stadium fell silent for a profoundly moving homage ahead of the contest as ten bells tolled in memory of the city's native Hatton, who died aged 46 in last month.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime disappeared, the quiet transitioned to a shout of "there's only one Hatton".
The action during the bout equaled the feeling. Arthur was sent to the canvas by a left hook flush on the chin in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored attire of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up quickly, noticeable damage developing under his orbital area as the duo exchanged powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the decision with official scores of draw, 115-113 and 115-112, but both fighters were determined and brave, all the qualities their inspiration Hatton represented.
