UK light heavyweight Buatsi secured a victory yet unimpressive start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding marring the action.
Two judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my finest display but how do champions respond? They win," the victor stated in the ringside as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers believed Parker, thirty-one, deserved the decision but the contender endured the second defeat of his 28-bout career.
Buatsi Secures Triumph in Uneventful Match
Buatsi's progress has stalled in recent times through contract problems and extended periods of lack of fights.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors prepared to overcommit in a tentative opening few rounds.
Parker, whose only previous defeat came at the lower weight class against his former opponent in two years ago after damaging his hand mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who operated with a solid defense to narrowly win the better rounds.
The spectators' acceptance diminished because of regular grabbing, with the challenger going down multiple occasions during clinches.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding local rivalry contest to capture the European light-heavyweight championship.
The venue grew hushed for a deeply poignant homage before the fight as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast match card in the city since Hatton's passing. As the concluding sound disappeared, the silence became a shout of "there's just one Ricky Hatton".
The action inside the ropes matched the emotion. Arthur was knocked down by a left hook flush on the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was injured soon, noticeable damage appearing under his face as the duo traded hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of 114-114, close margin and clearer margin, but each competitor were persistent and courageous, all the qualities their inspiration Hatton stood for.
