British light heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a match where the styles failed to mesh, with frequent clinching spoiling the contest.
Two judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my best performance but what defines champions? They succeed," Buatsi said in the ring as he was met by boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th pro success.
Several boxing experts thought Parker, thirty-one, deserved the decision but the British boxer experienced the second defeat of his 28-match professional journey.
Buatsi Claims Triumph in Uneventful Encounter
His career development has plateaued in the past few years through contract problems and extended periods of lack of fights.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with the two boxers willing to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance decreased amid constant clinching, with Parker falling to the floor multiple occasions during clinches.
Parker ended stronger, but several stanzas were challenging to assess. Nevertheless, former world champion Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester battle to capture the European light heavyweight title.
The stadium fell silent for a deeply poignant homage prior to the bout as a bell ceremony rang in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the premier televised match card in the city since the passing of Hatton. As the last bell faded, the stillness gave way to a chant of "there's only one Ricky Hatton".
The action inside the ropes equaled the emotion. Arthur was floored by a left hook directly to the face in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was damaged quickly, heavy swelling appearing under his orbital area as the duo landed hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur edged the verdict with judges scorecards of 114-114, narrow lead and 115-112, but each competitor were persistent and courageous, all the qualities their inspiration Hatton represented.
