UK light heavyweight Joshua Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a contest where the techniques failed to mesh, with excessive holding spoiling the contest.
The scoring judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They win," Buatsi commented in the ringside as he was met by boos from the audience.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers felt the challenger, thirty-one, should have been awarded the victory but the contender endured the second loss of his 28-bout professional journey.
Buatsi Secures Win in Slow Encounter
His career development has stalled in recent times through management issues and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - possibly in what seems like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter willing to take risks 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 making just his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, making life awkward for Buatsi, who operated with a solid defense to barely take the more effective rounds.
The crowd's patience diminished because of regular grabbing, with the challenger going down various instances during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, former world champion Carl Frampton - working with Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester fight to win the European light-heavyweight belt.
The venue became quiet for a extremely emotional tribute prior to the bout as a bell ceremony sounded in memory of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's death. As the last bell disappeared, the stillness transitioned to a shout of "there's just one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was knocked down by a left hook directly to the chin in the second round, rising shakily to his position on the count of eight.
But the Manchester United-red attire of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was marked up early, noticeable damage appearing under his eye as the duo exchanged punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of draw, 115-113 and 115-112, but the two athletes were determined and brave, exactly what their hero Hatton represented.
