UK light-heavyweight Buatsi earned a win yet disappointing start to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, landing some telling single shots in a contest where the techniques never quite clicked, with excessive holding disrupting the contest.
Two judges scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest performance but how do champions respond? They win," the victor said in the arena as he was faced disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the decision but the British boxer endured the second setback of his 28-match professional journey.
The Champion Secures Triumph in Slow Affair
The fighter's advancement has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers ready to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a solid defense to barely take the cleaner rounds.
The audience's tolerance diminished amid frequent holding, with the challenger falling to the floor various instances during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the co-main event, Arthur defeated Rea by close verdict in a terrific all-Manchester fight to capture the European light heavyweight championship.
The stadium fell silent for a deeply poignant memorial ahead of the contest as 10 bells sounded in memory of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the first major televised fight night in the city since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a roar of "there's only one Ricky Hatton".
The action inside the ropes matched the feeling. The eventual winner was knocked down by a left hook directly to the chin in the second round, rising shakily to his position on the referee's count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was injured quickly, noticeable damage appearing under his face as the duo landed hard punches.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur edged the verdict with official scores of draw, 115-113 and wider gap, but each competitor were persistent and brave, everything their inspiration Hatton represented.
