UK light heavyweight Joshua Buatsi earned a win yet unimpressive start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a majority decision in a tight ten-round contest at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches failed to mesh, with constant grabbing disrupting the fight.
The scoring judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest display but what defines champions? They win," Buatsi commented in the arena as he was met by boos from the audience.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th career victory.
Various analysts thought Parker, thirty-one, deserved the decision but the contender endured the second defeat of his 28-match professional journey.
Buatsi Claims Win in Uneventful Match
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a cagey initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in two years ago after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, causing problems for Buatsi, who operated with a tight guard to barely take the cleaner stanzas.
The audience's tolerance wore thin during regular grabbing, with Parker going down various instances during grappling.
Parker closed well, but many rounds were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester fight to win the European light-heavyweight belt.
The stadium fell silent for a extremely emotional tribute prior to the bout as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's passing. As the final chime ended, the silence became a shout of "there's just one Hatton".
The action inside the ropes matched the feeling. Arthur was knocked down by a powerful punch directly to the chin in the early round, rising shakily to his footing on the eight count.
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, heavy swelling developing under his orbital area as the fighters traded punishing blows.
The challenger, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur edged the decision with judges scorecards of even score, 115-113 and wider gap, but both fighters were persistent and courageous, all the qualities their hero the legend stood for.
