UK light-heavyweight Joshua Buatsi earned a win yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight 10-round contest at the boxing arena.
But the challenger landed significant shots, landing some telling single shots in a match where the techniques failed to mesh, with constant grabbing disrupting the contest.
The officials scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," Buatsi commented in the ring as he was faced boos from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts believed Parker, thirty-one, deserved the win but the contender endured the second defeat of his 28-bout professional journey.
Buatsi Secures Triumph in Dull Match
His career development has slowed in recent times through contract problems and extended periods of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to realize his potential - possibly in what seems like a final roll of the dice - under fresh management.
He started carefully, with the two boxers willing to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who utilized a solid defense to narrowly win the more effective periods.
The audience's tolerance wore thin because of constant clinching, with Parker falling to the floor several times during grappling.
The underdog ended stronger, but several stanzas were challenging to assess. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels All-Manchester Contest
In the secondary fight, Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight title.
The stadium fell silent for a deeply poignant memorial prior to the bout as a bell ceremony rang in remembrance of local hero Hatton, who succumbed 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the final chime faded, the quiet became a shout of "only one Ricky Hatton".
The competition inside the ropes matched the sentiment. Arthur was sent to the canvas by a significant blow flush on the face in the early round, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red shorts of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising developing under his eye as the fighters landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur barely secured the verdict with scoring tallies of even score, 115-113 and wider gap, but both fighters were relentless and courageous, everything their idol Hatton stood for.
