UK light-heavyweight Buatsi secured a victory yet underwhelming debut to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a split decision in a tight ten-round fight at the Manchester venue.
But Parker found success, delivering effective clean blows in a fight where the styles failed to mesh, with constant grabbing disrupting the fight.
The officials had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They prevail," the victor stated in the ring as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th pro success.
Several boxing experts felt the challenger, thirty-one, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Earns Win in Slow Match
The fighter's advancement has plateaued in recent times through promotional disputes and long spells of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after damaging his hand during the bout, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who worked behind a solid defense to edge the more effective periods.
The spectators' acceptance diminished amid frequent holding, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but multiple periods were difficult to score. Although, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Fuels Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent Manchester vs Manchester fight to win the European light heavyweight title.
The arena fell silent for a deeply poignant homage ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime faded, the silence became a chant of "there's only one Hatton".
The action inside the ropes equaled the feeling. Arthur was knocked down by a powerful punch flush on the face in the initial stage, rising shakily to his position on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured soon, noticeable damage forming under his eye as the pair traded punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
The victor narrowly won the verdict with judges scorecards of draw, 115-113 and 115-112, but both fighters were relentless and brave, exactly what their inspiration Hatton stood for.
