UK light-heavyweight Buatsi secured a victory yet unimpressive start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But the challenger found success, landing some telling clean blows in a match where the techniques failed to mesh, with excessive holding spoiling the fight.
The officials scored it 96-94 in favor to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the arena as he was met by negative reactions from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts thought Parker, 31, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-fight career.
Buatsi Claims Triumph in Slow Affair
His career development has stalled in recent times through management issues and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who operated with a solid defense to barely take the more effective rounds.
The audience's tolerance decreased because of regular grabbing, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to secure the European light-heavyweight title.
The arena became quiet for a profoundly moving tribute before the fight as a bell ceremony tolled in remembrance of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell disappeared, the stillness became a chant of "there's only one Hatton".
The fighting inside the ropes reflected the emotion. Arthur was knocked down by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his footing on the referee's count.
But the traditional red shorts of the fighter soon advanced again. On the opposite side, Rea - in rival City blue - was marked up quickly, noticeable damage developing under his face as the pair exchanged hard punches.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur narrowly won the result with scoring tallies of even score, narrow lead and 115-112, but the two athletes were relentless and brave, exactly what their idol the legend represented.
