UK light-heavyweight Buatsi earned a win yet disappointing debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the styles failed to mesh, with frequent clinching disrupting the action.
The scoring judges judged it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They succeed," the victor stated in the ring as he was faced negative reactions from the crowd.
The Londoner got back to winning after his initial pro loss - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second setback of his 28-fight career.
Buatsi Earns Triumph in Slow Match
The fighter's advancement has slowed in the past few years through management issues and long spells of inactivity.
Previously predicted as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a final roll of the dice - under fresh management.
He started carefully, with the two boxers prepared to take risks in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who utilized a solid defense to edge the more effective periods.
The audience's tolerance wore thin during constant clinching, with Parker going down multiple occasions during clinches.
The underdog ended stronger, but several stanzas were hard to judge. Although, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent local rivalry contest to secure the European light heavyweight title.
The venue grew hushed for a extremely emotional tribute ahead of the contest as 10 bells rang in remembrance of Manchester's own Ricky Hatton, who died aged 46 in September.
It was the first major televised fight night in the location since Hatton's death. As the last bell disappeared, the quiet became a chant of "there's only one Hatton".
The action within the ring equaled the feeling. The eventual winner was knocked down by a powerful punch directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was marked up soon, significant bruising developing under his orbital area as the pair landed punishing blows.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over twelve rounds.
The victor barely secured the verdict with judges scorecards of draw, close margin and clearer margin, but the two athletes were persistent and courageous, exactly what their hero the legend embodied.
