UK light heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a fight where the techniques never quite clicked, with excessive holding marring the contest.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They prevail," Buatsi commented in the ring as he was faced negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in last winter - to claim a twentieth professional win.
Some ringside observers believed the challenger, 31, merited the decision but the British boxer suffered the second loss of his 28-fight professional journey.
The Champion Claims Win in Dull Affair
His career development has plateaued in the past few years through management issues and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers willing to overcommit in a calculated opening few rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to narrowly win the cleaner stanzas.
The audience's tolerance wore thin during constant clinching, with Parker falling to the floor multiple occasions during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, 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 hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight championship.
The arena grew hushed for a profoundly moving memorial before the fight as a bell ceremony rang in remembrance of Manchester's own Hatton, who passed away 46 years old in recently.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime disappeared, the silence became a roar of "only one Ricky Hatton".
The fighting within the ring reflected the sentiment. Arthur was floored by a left hook flush on the chin in the early round, standing uncertainly to his footing on the eight count.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, Rea - in competitive blue - was injured soon, noticeable damage developing under his face as the pair exchanged punishing blows.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the result with judges scorecards of draw, close margin and wider gap, but the two athletes were persistent and full of heart, everything their idol Hatton embodied.
