British light-heavyweight Buatsi made a winning yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective single shots in a match where the approaches failed to mesh, with frequent clinching disrupting the fight.
The scoring judges had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best showing but what defines champions? They succeed," Buatsi commented in the ringside as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Several boxing experts felt the challenger, 31, deserved the decision but the contender suffered the second loss of his 28-fight career.
Buatsi Claims Win in Slow Affair
The fighter's advancement has plateaued in the past few years through contract problems and long spells of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors willing to take risks in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to barely take the more effective rounds.
The crowd's patience diminished because of constant clinching, with Parker falling to the floor multiple occasions during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the supporting bout, Arthur beat Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to capture the European light-heavyweight title.
The venue fell silent for a deeply poignant homage before the fight as 10 bells rang in honor of the city's native Ricky Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime faded, the silence gave way to a chant of "there's only one Ricky Hatton".
The action during the bout matched the feeling. Arthur was knocked down by a left hook directly to the face in the initial stage, standing uncertainly to his footing on the eight count.
But the traditional red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up soon, noticeable damage developing under his face as the duo exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and courageous, exactly what their hero the legend embodied.
