British light-heavyweight Joshua Buatsi earned a win yet unimpressive debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a close ten-round fight at the boxing arena.
But the challenger found success, landing some telling individual punches in a contest where the styles failed to mesh, with frequent clinching marring the contest.
The scoring judges judged it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my best showing but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with boos from the crowd.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to claim a 20th pro success.
Several boxing experts believed the challenger, 31, merited the decision but the British boxer experienced the second setback of his 28-match professional journey.
Buatsi Claims Triumph in Slow Match
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to edge the better stanzas.
The audience's tolerance wore thin during regular grabbing, with Parker going down multiple occasions during clinches.
Parker ended stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a excellent local rivalry contest to win the continental championship.
The arena fell silent for a deeply poignant homage ahead of the contest as 10 bells tolled in memory of the city's native Ricky Hatton, who died 46 years old in recently.
It was the premier televised match card in the location since Hatton's passing. As the concluding sound disappeared, the stillness gave way to a shout of "only one Ricky Hatton".
The action within the ring reflected the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up quickly, heavy swelling appearing under his eye as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the action hardly diminished over 12 rounds.
The victor edged the result with judges scorecards of even score, narrow lead and wider gap, but both fighters were determined and courageous, all the qualities their inspiration the legend embodied.
