British light-heavyweight Buatsi earned a win yet unimpressive debut to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close ten-round fight at the Manchester venue.
But the challenger had his moments, delivering effective clean blows in a fight where the approaches didn't complement each other, with frequent clinching disrupting the fight.
The officials scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They prevail," Buatsi said in the arena as he was faced negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in February - to claim a 20th career victory.
Various analysts thought the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Secures Victory in Slow Match
The fighter's advancement has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to achieve expectations - possibly in what feels like a last chance - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had kept it close, making life awkward for the favorite, who worked behind a solid defense to barely take the better rounds.
The spectators' acceptance diminished amid frequent holding, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. However, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a terrific local rivalry battle to win the continental title.
The venue grew hushed for a extremely emotional homage prior to the bout as a bell ceremony tolled in memory of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast match card in the location since Hatton's passing. As the final chime disappeared, the silence gave way to a shout of "only one Hatton".
The competition within the ring matched the sentiment. Arthur was sent to the canvas by a significant blow squarely on the face in the second round, standing uncertainly to his footing on the referee's count.
But the traditional red attire of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising developing under his orbital area as the fighters traded punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over the full distance.
The victor edged the result with judges scorecards of draw, close margin and clearer margin, but both fighters were relentless and full of heart, everything their inspiration the legend embodied.
