British light heavyweight Buatsi secured a victory yet disappointing debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight ten-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the approaches failed to mesh, with excessive holding spoiling the contest.
Two judges scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest performance but how do champions respond? They succeed," the victor said in the ring as he was met by boos from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to earn a 20th career victory.
Several boxing experts thought Parker, thirty-one, merited the decision but the Derbyshire fighter experienced the second defeat of his 28-fight career.
Buatsi Claims Victory in Uneventful Affair
The fighter's advancement has plateaued in recent years through promotional disputes and long spells of lack of fights.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors ready to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the better rounds.
The crowd's patience decreased amid constant clinching, with Parker slipping to the canvas various instances during clinches.
The underdog closed well, but many rounds were difficult to score. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by close verdict in a terrific all-Manchester contest to secure the European light heavyweight championship.
The stadium grew hushed for a extremely emotional memorial prior to the bout as a bell ceremony tolled in memory of local hero Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the city since Hatton's passing. As the last bell ended, the stillness gave way to a chant of "only one Ricky Hatton".
The fighting inside the ropes equaled the feeling. Arthur was floored by a significant blow directly to the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the traditional red equipment of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was marked up quickly, heavy swelling appearing under his eye as the duo traded punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
The victor narrowly won the result with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were relentless and brave, exactly what their idol Hatton stood for.
