British light heavyweight Joshua Buatsi earned a win yet underwhelming start to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round contest at the boxing arena.
But the challenger had his moments, landing some telling individual punches in a fight where the techniques didn't complement each other, with excessive holding disrupting the contest.
The officials scored it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They win," the victor stated in the arena as he was greeted with boos from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to claim a 20th career victory.
Various analysts believed Parker, 31, deserved the victory but the British boxer endured the second loss of his 28-fight career.
Buatsi Secures Victory in Dull Match
His career development has slowed in the past few years through contract problems and extended periods of lack of fights.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with the two boxers willing to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who utilized a tight guard to narrowly win the more effective rounds.
The spectators' acceptance decreased because of constant clinching, with the challenger falling to the floor various instances during clinches.
Parker finished the stronger, but multiple periods were hard to judge. However, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific all-Manchester battle to win the continental championship.
The arena grew hushed for a deeply poignant memorial before the fight as a bell ceremony rang in remembrance of Manchester's own Hatton, who passed away at age 46 in September.
It was the initial significant broadcast match card in the location since the passing of Hatton. As the last bell disappeared, the quiet gave way to a shout of "there's only one Hatton".
The competition during the bout reflected the feeling. Arthur was knocked down by a significant blow flush on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the red-colored attire of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was marked up soon, significant bruising forming under his orbital area as the pair exchanged punishing blows.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the verdict with scoring tallies of draw, close margin and 115-112, but both fighters were determined and full of heart, all the qualities their idol the legend embodied.
