British light-heavyweight Joshua Buatsi secured a victory yet underwhelming debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a contest where the styles failed to mesh, with excessive holding marring the fight.
Two judges scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They prevail," the victor said in the ringside as he was met by boos from the audience.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, deserved the decision but the contender endured the second defeat of his 28-bout professional journey.
The Champion Earns Victory in Slow Match
Buatsi's progress has slowed in recent years through contract problems and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who operated with a tight guard to barely take the cleaner stanzas.
The audience's tolerance diminished during constant clinching, with Parker going down several times during close exchanges.
Parker ended stronger, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor progresses, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Arthur beat Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to capture the European light heavyweight championship.
The arena grew hushed for a profoundly moving memorial prior to the bout as a bell ceremony tolled in remembrance of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the concluding sound disappeared, the quiet became a shout of "there's only one Hatton".
The competition inside the ropes equaled the feeling. Arthur was sent to the canvas by a significant blow flush on the chin in the second round, standing uncertainly to his footing on the count of eight.
But the traditional red attire of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was injured soon, significant bruising forming under his face as the pair landed hard punches.
The challenger, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action rarely dropped over twelve rounds.
The victor edged the verdict with judges scorecards of even score, close margin and wider gap, but both fighters were persistent and courageous, all the qualities their inspiration the legend represented.
