UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round bout at the boxing arena.
But Parker found success, delivering effective clean blows in a fight where the techniques didn't complement each other, with excessive holding marring the action.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my finest display but what defines champions? They win," Buatsi stated in the ringside as he was met by disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Various analysts felt Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-fight career.
The Champion Earns Win in Uneventful Match
The fighter's advancement has slowed in the past few years through management issues and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with the two boxers willing to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a tight guard to narrowly win the better periods.
The crowd's patience wore thin because of regular grabbing, with Parker falling to the floor multiple occasions during grappling.
Parker ended stronger, but several stanzas were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the co-main event, Arthur defeated Rea by split-decision in a terrific local rivalry fight to win the European light-heavyweight title.
The venue fell silent for a profoundly moving tribute prior to the bout as a bell ceremony sounded in memory of local hero Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast fight night in the urban center since Hatton's passing. As the last bell ended, the stillness transitioned to a shout of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the sentiment. Arthur was floored by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was injured soon, heavy swelling forming under his orbital area as the duo landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor narrowly won the result with scoring tallies of 114-114, close margin and clearer margin, but both fighters were relentless and courageous, all the qualities their hero the legend embodied.
