British light-heavyweight Joshua Buatsi earned a win yet unimpressive start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close ten-round bout at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a match where the techniques didn't complement each other, with excessive holding disrupting the action.
The scoring judges scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but what defines champions? They succeed," Buatsi said in the ringside as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers believed Parker, 31, merited the decision but the contender suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Slow Affair
Buatsi's progress has stalled in recent years through management issues and considerable time of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who worked behind a solid defense to narrowly win the more effective rounds.
The spectators' acceptance diminished because of regular grabbing, with Parker slipping to the canvas various instances during grappling.
Parker closed well, but multiple periods were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - judging by this showing - still looks steep.
Hatton Homage Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight championship.
The stadium fell silent for a deeply poignant tribute before the fight as 10 bells rang in memory of Manchester's own Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast match card in the urban center since Hatton's death. As the last bell disappeared, the silence gave way to a chant of "there's only one Hatton".
The action within the ring equaled the sentiment. Arthur was floored by a left hook squarely on the face in the second round, rising shakily to his feet on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured early, heavy swelling appearing under his eye as the pair exchanged hard punches.
Rea, 27, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action rarely dropped over twelve rounds.
The victor edged the verdict with scoring tallies of draw, narrow lead and clearer margin, but each competitor were relentless and brave, everything their inspiration the legend represented.
