UK light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a close ten-round contest at the boxing arena.
But Parker landed significant shots, landing some telling clean blows in a contest where the styles failed to mesh, with frequent clinching spoiling the fight.
Two judges had it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best display but what do champions do? They win," Buatsi stated in the arena as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, merited the win but the contender endured the second defeat of his 28-bout professional journey.
The Champion Secures Triumph in Dull Encounter
Buatsi's progress has plateaued in the past few years through contract problems and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors willing to be aggressive in a calculated initial rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who utilized a protective stance to edge the more effective stanzas.
The audience's tolerance diminished during regular grabbing, with the challenger slipping to the canvas several times during grappling.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Arthur overcame Bradley Rea by divided decision in a outstanding local rivalry battle to win the continental championship.
The stadium became quiet for a extremely emotional homage ahead of the contest as a bell ceremony sounded in honor of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the final chime disappeared, the stillness gave way to a roar of "there's just one Hatton".
The fighting inside the ropes matched the feeling. The eventual winner was knocked down by a powerful punch squarely on the face in the early round, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was damaged soon, significant bruising appearing under his orbital area as the pair traded hard punches.
The challenger, 27, landed power punches but Arthur, 34, matched the intensity with stinging uppercuts as the action rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of even score, 115-113 and wider gap, but each competitor were determined and brave, exactly what their hero Hatton embodied.
