UK light heavyweight Buatsi secured a victory yet underwhelming debut to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a judges' verdict in a competitive 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a fight where the styles failed to mesh, with frequent clinching spoiling the fight.
The scoring judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best display but how do champions respond? They win," the victor commented in the ring as he was met by boos from the audience.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, deserved the win but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
The Champion Secures Triumph in Uneventful Match
The fighter's advancement has stalled in recent years through promotional disputes and long spells of inactivity.
Previously predicted as the next star of UK boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a tentative opening few rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to narrowly win the better periods.
The audience's tolerance decreased amid regular grabbing, with Parker going down several times during grappling.
Parker closed well, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester contest to win the continental title.
The stadium fell silent for a profoundly moving tribute prior to the bout as 10 bells tolled in honor of local hero Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime ended, the stillness gave way to a roar of "there's only one Hatton".
The fighting within the ring matched the emotion. Arthur was floored by a significant blow squarely on the jaw in the second round, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was injured soon, significant bruising developing under his face as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, fought fire with fire with effective uppercuts as the action hardly diminished over twelve rounds.
The victor edged the result with judges scorecards of draw, 115-113 and 115-112, but the two athletes were persistent and brave, everything their hero Hatton represented.
