British light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round fight at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a contest where the styles never quite clicked, with excessive holding disrupting the contest.
Two judges scored it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They prevail," Buatsi stated in the ringside as he was greeted with boos from the audience.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers felt Parker, thirty-one, merited the victory but the contender experienced the second defeat of his 28-bout career.
Buatsi Claims Victory in Slow Affair
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He started carefully, with the two boxers ready to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for the favorite, who worked behind a solid defense to narrowly win the more effective periods.
The audience's tolerance wore thin during frequent holding, with Parker falling to the floor various instances during grappling.
Parker finished the stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a terrific local rivalry fight to secure the continental belt.
The arena fell silent for a profoundly moving homage prior to the bout as ten bells tolled in memory of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime disappeared, the quiet transitioned to a shout of "there's just one Hatton".
The action inside the ropes reflected the feeling. Arthur was knocked down by a significant blow directly to the face in the initial stage, getting up unsteadily to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged soon, noticeable damage forming under his face as the fighters traded hard punches.
Rea, 27, landed power punches but Arthur, thirty-four, matched the intensity with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the result with official scores of draw, 115-113 and clearer margin, but the two athletes were persistent and courageous, all the qualities their hero Hatton stood for.
