UK light-heavyweight Joshua Buatsi earned a win yet disappointing start to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a close 10-round contest at the Manchester venue.
But Parker had his moments, landing some telling single shots in a match where the styles failed to mesh, with excessive holding marring the fight.
The officials scored it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest performance but how do champions respond? They succeed," Buatsi said in the arena as he was faced disapproval from the audience.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, should have been awarded the win but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Slow Match
His career development has slowed in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to be aggressive in a cagey initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to narrowly win the more effective periods.
The crowd's patience diminished amid frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker finished the stronger, but several stanzas were difficult to score. Although, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester battle to secure the continental belt.
The venue became quiet for a extremely emotional tribute prior to the bout as a bell ceremony sounded in memory of the city's native Hatton, who died aged 46 in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the final chime disappeared, the silence became a roar of "there's only one Hatton".
The competition inside the ropes equaled the feeling. Arthur was sent to the canvas by a powerful punch flush on the chin in the early round, getting up unsteadily to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. Across the ring, the opponent - in rival City blue - was marked up early, heavy swelling appearing under his orbital area as the pair landed hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action hardly diminished over the full distance.
The victor edged the decision with official scores of draw, narrow lead and wider gap, but each competitor were relentless and courageous, everything their inspiration the legend embodied.
