UK light heavyweight Buatsi made a winning yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a close 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective single shots in a contest where the approaches never quite clicked, with frequent clinching disrupting the fight.
The scoring judges scored it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They succeed," Buatsi stated in the ringside as he was faced boos from the audience.
The British fighter got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Several boxing experts believed Parker, thirty-one, merited the decision but the Derbyshire fighter suffered the second loss of his 28-bout career.
Buatsi Secures Triumph in Uneventful Match
The fighter's advancement has slowed in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the future of UK boxing, he now is positioned trying to achieve expectations - possibly in what feels like a last chance - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in two years ago after suffering a hand injury during the bout, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to narrowly win the more effective rounds.
The crowd's patience decreased during frequent holding, with the challenger falling to the floor multiple occasions during clinches.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just a single round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian champion Dmitry Bivol ruling the division, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to win the continental title.
The venue grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in remembrance of Manchester's own Hatton, who passed away 46 years old in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the last bell faded, the quiet gave way to a roar of "there's just one Hatton".
The competition during the bout matched the emotion. Arthur was knocked down by a left hook flush on the jaw in the second round, standing uncertainly to his feet on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was marked up quickly, significant bruising appearing under his orbital area as the pair landed punishing blows.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over the full distance.
Arthur edged the decision with scoring tallies of draw, close margin and wider gap, but both fighters were persistent and full of heart, exactly what their inspiration the legend embodied.
