British light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful clean blows in a match where the techniques didn't complement each other, with excessive holding disrupting the fight.
Two judges judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They prevail," the victor said in the ring as he was greeted with boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a 20th pro success.
Several boxing experts believed Parker, 31, deserved the win but the contender experienced the second loss of his 28-fight professional journey.
The Champion Secures Win in Uneventful Affair
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He started carefully, with both competitors ready to take risks in a cagey 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 making just his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who utilized a protective stance to edge the more effective periods.
The audience's tolerance wore thin because of frequent holding, with the challenger falling to the floor several times during clinches.
The underdog ended stronger, but many rounds were challenging to assess. However, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still seems difficult.
Hatton Memorial Inspires Manchester Derby Fight
In the supporting bout, Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester fight to capture the continental title.
The venue grew hushed for a extremely emotional tribute prior to the bout as 10 bells tolled in memory of local hero Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the urban center since the passing of Hatton. As the concluding sound disappeared, the silence became a chant of "only one Ricky Hatton".
The action within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow flush on the chin in the second round, rising shakily to his feet on the eight count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was damaged early, heavy swelling forming under his orbital area as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor edged the result with official scores of even score, 115-113 and wider gap, but both fighters were relentless and full of heart, exactly what their inspiration Hatton stood for.
