UK light-heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a close ten-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a match where the styles failed to mesh, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my greatest showing but how do champions respond? They win," Buatsi stated in the arena as he was greeted with negative reactions from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, 31, merited the victory but the Derbyshire fighter endured the second defeat of his 28-fight career.
The Champion Claims Triumph in Slow Match
Buatsi's progress has plateaued in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now stands trying to fulfil that early promise - perhaps in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter willing to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to edge the cleaner periods.
The audience's tolerance decreased because of regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Tribute Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to capture the European light heavyweight championship.
The stadium grew hushed for a extremely emotional homage ahead of the contest as ten bells sounded in honor of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised match card in the city since Hatton's passing. As the final chime disappeared, the quiet gave way to a roar of "only one Ricky Hatton".
The competition within the ring reflected the feeling. The eventual winner was knocked down by a significant blow flush on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, the opponent - in competitive blue - was damaged soon, heavy swelling forming under his face as the duo exchanged hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur edged the verdict with judges scorecards of even score, 115-113 and clearer margin, but each competitor were relentless and full of heart, exactly what their hero the legend embodied.
