British light-heavyweight Joshua Buatsi secured a victory yet underwhelming beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive ten-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a contest where the techniques never quite clicked, with frequent clinching marring the fight.
Two judges had it 96-94 for to Buatsi, with the final judge a 95-95.
"It wasn't my greatest showing but what do champions do? They prevail," the victor said in the arena as he was faced negative reactions from the spectators.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers thought the challenger, thirty-one, merited the win but the British boxer endured the second loss of his 28-match career.
Buatsi Earns Victory in Dull Match
The fighter's advancement has stalled in recent years through management issues and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with the two boxers willing to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who operated with a solid defense to narrowly win the better periods.
The crowd's patience decreased amid constant clinching, with the challenger falling to the floor multiple occasions during grappling.
The underdog closed well, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester battle to secure the European light heavyweight belt.
The stadium grew hushed for a extremely emotional memorial prior to the bout as 10 bells tolled in honor of the city's native Ricky Hatton, who died aged 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the last bell disappeared, the stillness gave way to a chant of "only one Ricky Hatton".
The action within the ring reflected the feeling. Arthur was floored by a powerful punch squarely on the chin in the initial stage, rising shakily to his position on the count of eight.
But the traditional red equipment of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was injured soon, noticeable damage appearing under his face as the duo traded punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over 12 rounds.
The victor narrowly won the result with scoring tallies of draw, narrow lead and 115-112, but the two athletes were determined and courageous, everything their inspiration Hatton stood for.
