British light heavyweight Joshua Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a close ten-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with constant grabbing spoiling the action.
The scoring judges scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They prevail," Buatsi said in the ring as he was greeted with boos from the spectators.
The British fighter bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts believed Parker, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Dull Encounter
Buatsi's progress has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of UK 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 both competitors willing to overcommit in a tentative early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the cleaner rounds.
The crowd's patience diminished because of frequent holding, with the challenger going down multiple occasions during close exchanges.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels All-Manchester Battle
In the supporting bout, Arthur overcame Rea by divided decision in a terrific Manchester vs Manchester battle to capture the European light-heavyweight title.
The venue became quiet for a extremely emotional memorial ahead of the contest as ten bells rang in honor of Manchester's own Ricky Hatton, who died 46 years old in September.
It was the first major televised match card in the city since Hatton's passing. As the final chime ended, the stillness transitioned to a chant of "there's just one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was sent to the canvas by a powerful punch flush on the face in the early round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was injured early, significant bruising appearing under his eye as the pair traded punishing blows.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over twelve rounds.
The victor edged the decision with judges scorecards of draw, close margin and 115-112, but the two athletes were persistent and courageous, all the qualities their hero Hatton stood for.
