UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a competitive 10-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a contest where the approaches failed to mesh, with frequent clinching disrupting the contest.
The scoring judges judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but how do champions respond? They succeed," the victor stated in the ringside as he was faced boos from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Various analysts believed the challenger, 31, deserved the decision but the British boxer suffered the second defeat of his 28-match professional journey.
Buatsi Claims Triumph in Uneventful Affair
His career development has stalled in recent years through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He opened tentatively, with neither fighter prepared to overcommit in a cagey early stages.
The challenger, 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 appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who utilized a tight guard to narrowly win the more effective stanzas.
The crowd's patience diminished during frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates All-Manchester Fight
In the co-main event, Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester contest to win the European light-heavyweight belt.
The venue fell silent for a deeply poignant homage before the fight as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the final chime disappeared, the quiet became a shout of "there's just one Hatton".
The action during the bout matched the emotion. The eventual winner was sent to the canvas by a significant blow directly to the face in the early round, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up soon, heavy swelling forming under his face as the duo exchanged punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, matched the intensity with powerful upward punches as the pace seldom decreased over the full distance.
The victor edged the decision with official scores of even score, 115-113 and clearer margin, but the two athletes were determined and brave, all the qualities their inspiration Hatton represented.
