British light heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight ten-round bout at the boxing arena.
But the challenger found success, connecting with powerful individual punches in a contest where the techniques never quite clicked, with constant grabbing marring the contest.
Two judges judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They win," the victor stated in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, 31, merited the decision but the Derbyshire fighter endured the second loss of his 28-match career.
Buatsi Claims Triumph in Dull Match
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter prepared to be aggressive in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for Buatsi, who utilized a protective stance to narrowly win the cleaner periods.
The audience's tolerance decreased during constant clinching, with Parker falling to the floor various instances during close exchanges.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Homage Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific local rivalry fight to capture the continental belt.
The arena fell silent for a deeply poignant homage prior to the bout as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the location since Hatton's passing. As the concluding sound faded, the silence gave way to a chant of "only one Hatton".
The competition during the bout reflected the feeling. The eventual winner was floored by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his eye as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the decision with judges scorecards of draw, close margin and 115-112, but both fighters were determined and courageous, exactly what their idol Hatton embodied.
