UK light heavyweight Joshua Buatsi earned a win yet disappointing beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close ten-round fight at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a contest where the approaches failed to mesh, with constant grabbing marring the fight.
The officials scored it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best performance but what do champions do? They prevail," the victor commented in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers thought the challenger, 31, deserved the victory but the contender suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Affair
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter ready to be aggressive in a tentative initial rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after breaking his hand in the contest, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to barely take the more effective periods.
The audience's tolerance diminished amid frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Homage Fuels Local Rivalry Battle
In the secondary fight, Arthur beat Rea by close verdict in a excellent local rivalry contest to capture the continental belt.
The arena grew hushed for a profoundly moving memorial ahead of the contest as ten bells sounded in remembrance of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the city since the passing of Hatton. As the last bell disappeared, the silence became a roar of "there's just one Hatton".
The fighting inside the ropes equaled the feeling. The eventual winner was knocked down by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the red-colored attire of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was marked up quickly, significant bruising developing under his face as the duo landed powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the result with judges scorecards of even score, narrow lead and clearer margin, but the two athletes were relentless and courageous, exactly what their hero the legend stood for.
