UK light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to his career 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 ten-round contest at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a match where the techniques didn't complement each other, with frequent clinching disrupting the action.
The scoring judges had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest performance but what defines champions? They win," Buatsi stated in the arena as he was met by negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, 31, merited the decision but the contender experienced the second setback of his 28-fight professional journey.
The Champion Claims Win in Slow Match
His career development has plateaued in recent times through promotional disputes and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now stands trying to realize his potential - perhaps in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with neither fighter ready to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the more effective stanzas.
The crowd's patience decreased during regular grabbing, with Parker going down multiple occasions during grappling.
The underdog closed well, but many rounds were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Arthur beat Rea by divided decision in a terrific all-Manchester fight to secure the European light-heavyweight title.
The arena fell silent for a deeply poignant tribute prior to the bout as ten bells sounded in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the last bell faded, the stillness became a chant of "only one Ricky Hatton".
The competition during the bout equaled the feeling. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his feet on the count of eight.
But the red-colored attire of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was marked up soon, heavy swelling appearing under his face as the duo exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the action seldom decreased over the full distance.
Arthur barely secured the verdict with scoring tallies of even score, narrow lead and wider gap, but the two athletes were persistent and courageous, all the qualities their idol Hatton represented.
