British light heavyweight Joshua Buatsi secured a victory yet unimpressive beginning to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a close 10-round bout at the boxing arena.
But Parker landed significant shots, landing some telling single shots in a contest where the techniques never quite clicked, with excessive holding spoiling the contest.
The officials scored it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They win," the victor stated in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter endured the second setback of his 28-match professional journey.
Buatsi Secures Triumph in Dull Match
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to edge the better rounds.
The crowd's patience wore thin during regular grabbing, with Parker falling to the floor several times during close exchanges.
Parker closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Tribute Motivates Manchester Derby Battle
In the co-main event, Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to win the European light heavyweight championship.
The stadium fell silent for a deeply poignant tribute prior to the bout as ten bells sounded in honor of Manchester's own Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the city since the passing of Hatton. As the concluding sound faded, the stillness became a roar of "there's just one Hatton".
The action during the bout reflected the emotion. Arthur was knocked down by a powerful punch directly to the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was marked up soon, noticeable damage appearing under his orbital area as the duo landed punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
The victor narrowly won the verdict with judges scorecards of even score, narrow lead and wider gap, but each competitor were relentless and brave, all the qualities their inspiration Hatton stood for.
