British light heavyweight Buatsi made a winning yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the techniques failed to mesh, with constant grabbing marring the action.
The officials scored it 96-94 for to the winner, with the final judge a even score.
"It wasn't my finest display but how do champions respond? They succeed," the victor commented in the ringside as he was greeted with disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th career victory.
Various analysts thought Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second defeat of his 28-fight career.
The Champion Secures Victory in Dull Match
The fighter's advancement has slowed in the past few years through contract problems and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to fulfil that early promise - maybe in what seems like a last chance - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the halfway stage, Parker had kept it close, causing problems for the favorite, who operated with a protective stance to edge the more effective rounds.
The spectators' acceptance diminished during frequent holding, with Parker slipping to the canvas various instances during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Battle
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific local rivalry contest to win the European light-heavyweight title.
The venue grew hushed for a extremely emotional homage before the fight as 10 bells rang in memory of Manchester's own Hatton, who died at age 46 in recently.
It was the premier televised match card in the city since Hatton's death. As the concluding sound ended, the silence gave way to a chant of "only one Hatton".
The action during the bout equaled the sentiment. Arthur was floored by a significant blow directly to the jaw in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up quickly, heavy swelling developing under his eye as the pair landed hard punches.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over twelve rounds.
The victor edged the result with scoring tallies of even score, narrow lead and 115-112, but both fighters were determined and brave, all the qualities their idol the legend represented.
