UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a close ten-round fight at the boxing arena.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles failed to mesh, with constant grabbing disrupting the contest.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They win," the victor stated in the arena as he was met by boos from the crowd.
The Londoner bounced back after his maiden career setback - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers thought the challenger, 31, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-match career.
The Champion Claims Win in Slow Encounter
The fighter's advancement has stalled in recent years through management issues and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to realize his potential - possibly in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with both competitors prepared to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who operated with a tight guard to narrowly win the cleaner periods.
The audience's tolerance wore thin during regular grabbing, with the challenger falling to the floor several times during grappling.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding local rivalry battle to capture the European light heavyweight belt.
The stadium grew hushed for a extremely emotional homage before the fight as ten bells tolled in honor of the city's native Ricky Hatton, who succumbed aged 46 in September.
It was the premier televised fight night in the location since the passing of Hatton. As the final chime faded, the silence transitioned to a chant of "there's just one Ricky Hatton".
The action within the ring equaled the sentiment. The eventual winner was knocked down by a powerful punch squarely on the face in the early round, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up quickly, significant bruising forming under his face as the pair landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity hardly diminished over the full distance.
The victor edged the decision with official scores of even score, close margin and clearer margin, but both fighters were persistent and courageous, everything their hero Hatton represented.
