UK light heavyweight Joshua Buatsi made a winning yet disappointing debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a fight where the approaches never quite clicked, with excessive holding spoiling the fight.
Two judges had it 96-94 for to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They win," the victor commented in the arena as he was greeted with negative reactions from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, should have been awarded the victory but the contender experienced the second loss of his 28-bout career.
Buatsi Secures Triumph in Dull Match
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He started carefully, with both competitors prepared to overcommit in a cagey early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second fight at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for Buatsi, who worked behind a protective stance to narrowly win the more effective rounds.
The audience's tolerance wore thin during constant clinching, with Parker going down multiple occasions during grappling.
Parker ended stronger, but multiple periods were difficult to score. Although, previous titleholder Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Tribute Inspires Local Rivalry Fight
In the secondary fight, Arthur defeated Bradley Rea by divided decision in a outstanding Manchester vs Manchester fight to secure the European light heavyweight championship.
The venue became quiet for a deeply poignant memorial before the fight as ten bells sounded in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the concluding sound disappeared, the silence became a shout of "there's only one Hatton".
The competition inside the ropes equaled the emotion. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red attire of Arthur soon advanced again. On the opposite side, Rea - in competitive blue - was marked up soon, significant bruising appearing under his eye as the fighters landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over the full distance.
The victor narrowly won the result with scoring tallies of draw, narrow lead and 115-112, but both fighters were persistent and courageous, everything their hero the legend represented.
