British light heavyweight Buatsi earned a win yet underwhelming beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful individual punches in a match where the styles didn't complement each other, with constant grabbing disrupting the action.
The officials judged it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best display but how do champions respond? They succeed," Buatsi stated in the arena as he was met by boos from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt the challenger, 31, deserved the win but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Secures Victory in Dull Encounter
The fighter's advancement has slowed in the past few years through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter prepared to take risks in a tentative early stages.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for the favorite, who operated with a protective stance to barely take the more effective stanzas.
The audience's tolerance decreased amid frequent holding, with the challenger falling to the floor various instances during clinches.
Parker closed well, but many rounds were hard to judge. However, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor advances, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding local rivalry battle to secure the continental championship.
The venue fell silent for a extremely emotional homage before the fight as a bell ceremony sounded in remembrance of local hero Hatton, who died at age 46 in recently.
It was the first major televised match card in the urban center since Hatton's death. As the final chime ended, the silence transitioned to a shout of "only one Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a left hook directly to the jaw in the early round, rising shakily to his footing on the referee's count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was marked up early, heavy swelling developing under his face as the fighters traded hard punches.
The challenger, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur edged the decision with official scores of 114-114, narrow lead and clearer margin, but the two athletes were relentless and brave, exactly what their inspiration the legend stood for.
