UK light heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close ten-round bout at the boxing arena.
But Parker had his moments, delivering effective individual punches in a fight where the styles failed to mesh, with excessive holding spoiling the contest.
The scoring judges had it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest display but how do champions respond? They win," the victor said in the arena as he was met by 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 thought Parker, 31, merited the win but the contender suffered the second defeat of his 28-match professional journey.
Buatsi Secures Win in Uneventful Match
Buatsi's progress has stalled in recent years through contract problems and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a last chance - under recent support.
He started carefully, with the two boxers prepared to overcommit in a tentative opening few rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after breaking his hand during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who utilized a solid defense to edge the cleaner stanzas.
The audience's tolerance wore thin amid frequent holding, with Parker falling to the floor various instances during grappling.
The underdog finished the stronger, but many rounds were challenging to assess. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Arthur overcame Rea by divided decision in a excellent all-Manchester battle to win the European light heavyweight title.
The venue fell silent for a extremely emotional memorial prior to the bout as ten bells sounded in memory of Manchester's own Ricky Hatton, who passed away 46 years old in last month.
It was the first major televised match card in the location since the passing of Hatton. As the final chime faded, the silence gave way to a chant of "there's only one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was floored by a powerful punch squarely on the jaw in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was injured quickly, significant bruising forming under his face as the duo landed punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the pace seldom decreased over the full distance.
The victor edged the verdict with official scores of draw, narrow lead and wider gap, but each competitor were relentless and courageous, exactly what their inspiration Hatton embodied.
