British light heavyweight Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a tight 10-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a match where the techniques failed to mesh, with excessive holding disrupting the fight.
The scoring judges had it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my best showing but what defines champions? They win," the victor commented in the arena as he was greeted with disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Various analysts thought the challenger, thirty-one, deserved the victory but the British boxer suffered the second defeat of his 28-bout career.
The Champion Secures Triumph in Uneventful Match
The fighter's advancement has stalled in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to achieve expectations - perhaps in what feels like a last chance - under new promotional backing.
He began cautiously, with both competitors ready to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to narrowly win the cleaner periods.
The crowd's patience diminished during constant clinching, with Parker falling to the floor multiple occasions during clinches.
Parker closed well, but several stanzas were difficult to score. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent local rivalry fight to secure the European light heavyweight title.
The stadium fell silent for a deeply poignant tribute prior to the bout as a bell ceremony rang in memory of the city's native Hatton, who died at age 46 in last month.
It was the premier televised boxing event in the city since Hatton's death. As the last bell ended, the silence transitioned to a shout of "only one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a significant blow flush on the chin in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was damaged early, heavy swelling forming under his face as the duo exchanged punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over twelve rounds.
The victor narrowly won the result with judges scorecards of even score, close margin and 115-112, but both fighters were relentless and courageous, all the qualities their inspiration the legend represented.
