British light-heavyweight Buatsi earned a win yet underwhelming debut to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a match where the techniques never quite clicked, with excessive holding disrupting the action.
The scoring judges had it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," the victor stated in the ring as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Several boxing experts felt Parker, 31, deserved the decision but the contender suffered the second setback of his 28-fight professional journey.
Buatsi Claims Victory in Dull Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and long spells of lack of fights.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors ready to overcommit in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the cleaner periods.
The spectators' acceptance diminished because of frequent holding, with the challenger going down various instances during clinches.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by split-decision in a terrific local rivalry fight to win the continental belt.
The arena fell silent for a deeply poignant homage before the fight as a bell ceremony sounded in memory of the city's native Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell ended, the stillness gave way to a shout of "there's only one Hatton".
The fighting within the ring reflected the feeling. The eventual winner was floored by a left hook squarely on the jaw in the early round, standing uncertainly to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in rival City blue - was injured soon, significant bruising appearing under his face as the pair landed punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over the full distance.
Arthur edged the result with judges scorecards of draw, close margin and 115-112, but each competitor were persistent and full of heart, exactly what their inspiration Hatton represented.
