British light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight ten-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the approaches never quite clicked, with constant grabbing disrupting the action.
The scoring judges had it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but what do champions do? They win," the victor said in the arena as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Several boxing experts believed the challenger, 31, deserved the decision but the Derbyshire fighter experienced the second setback of his 28-fight professional journey.
The Champion Claims Triumph in Dull Match
The fighter's advancement has slowed in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after damaging his hand in the contest, was participating in his second bout at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to barely take the better periods.
The audience's tolerance decreased amid regular grabbing, with the challenger falling to the floor multiple occasions during grappling.
Parker ended stronger, but many rounds were difficult to score. However, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Fight
In the supporting bout, Arthur overcame Rea by divided decision in a excellent all-Manchester battle to secure the European light heavyweight championship.
The venue fell silent for a extremely emotional homage ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised match card in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a roar of "only one Hatton".
The action within the ring reflected the sentiment. The eventual winner was floored by a left hook flush on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, Rea - in competitive blue - was injured early, heavy swelling appearing under his face as the pair traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor edged the verdict with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were relentless and brave, exactly what their hero the legend embodied.
