UK light heavyweight Joshua Buatsi made a winning yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a fight where the styles never quite clicked, with frequent clinching marring the contest.
The officials had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi stated in the ringside as he was greeted with disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers felt the challenger, thirty-one, merited the victory but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Dull Affair
Buatsi's progress has plateaued in recent years through contract problems and long spells of inactivity.
Once tipped as the future of British boxing, he now is positioned trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a tentative initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who utilized a protective stance to barely take the cleaner stanzas.
The crowd's patience diminished amid constant clinching, with Parker slipping to the canvas multiple occasions during close exchanges.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Inspires Manchester Derby Battle
In the supporting bout, Arthur beat Rea by close verdict in a outstanding local rivalry fight to win the European light heavyweight championship.
The arena fell silent for a deeply poignant memorial prior to the bout as ten bells rang in honor of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the city since the passing of Hatton. As the concluding sound ended, the silence became a chant of "only one Hatton".
The action during the bout reflected the feeling. The eventual winner was knocked down by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was damaged quickly, significant bruising appearing under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action hardly diminished over twelve rounds.
Arthur edged the result with judges scorecards of 114-114, close margin and 115-112, but both fighters were relentless and brave, everything their hero the legend stood for.
