British light-heavyweight Joshua Buatsi made a winning yet disappointing 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 competitive 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a match where the styles never quite clicked, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my best display but what defines champions? They win," Buatsi stated in the ring as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts believed Parker, 31, should have been awarded the decision but the British boxer suffered the second defeat of his 28-bout career.
Buatsi Claims Win in Uneventful Affair
Buatsi's progress has slowed in the past few years through management issues and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what feels like a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second appearance at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the cleaner stanzas.
The audience's tolerance diminished because of frequent holding, with Parker falling to the floor various instances during grappling.
The underdog finished the stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor progresses, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Arthur beat Bradley Rea by divided decision in a excellent local rivalry contest to capture the continental belt.
The venue fell silent for a deeply poignant tribute before the fight as 10 bells rang in memory of local hero Ricky Hatton, who passed away at age 46 in September.
It was the first major televised fight night in the location since Hatton's passing. As the final chime ended, the quiet transitioned to a shout of "only one Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was sent to the canvas by a powerful punch flush on the chin in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his orbital area as the fighters landed powerful shots.
Rea, 27, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the decision with scoring tallies of 114-114, narrow lead and 115-112, but the two athletes were relentless and brave, everything their hero Hatton stood for.
