British light heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a points victory over Zach Parker in Manchester.
The Olympic medal winner's activity level likely gained him a majority decision in a tight 10-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a contest where the techniques failed to mesh, with constant grabbing spoiling the action.
The scoring judges had it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my finest display but how do champions respond? They win," the victor said in the ringside as he was met by disapproval from the spectators.
The Londoner got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Several boxing experts felt Parker, thirty-one, merited the victory but the British boxer endured the second loss of his 28-match career.
Buatsi Secures Triumph in Dull Match
Buatsi's progress has slowed in recent years through management issues and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to realize his potential - maybe in what seems like a last chance - under fresh management.
He opened tentatively, with both competitors prepared to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, creating difficulties for the favorite, who utilized a protective stance to edge the better rounds.
The spectators' acceptance decreased because of regular grabbing, with Parker falling to the floor various instances during close exchanges.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, awarding Buatsi just one round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Arthur overcame Rea by split-decision in a terrific Manchester vs Manchester contest to win the European light heavyweight title.
The venue became quiet for a deeply poignant tribute ahead of the contest as ten bells sounded in honor of local hero Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised match card in the urban center since the passing of Hatton. As the last bell ended, the quiet transitioned to a shout of "only one Hatton".
The fighting during the bout matched the feeling. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the second round, rising shakily to his feet on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was marked up quickly, heavy swelling appearing under his eye as the fighters traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over the full distance.
The victor edged the decision with judges scorecards of even score, 115-113 and clearer margin, but the two athletes were determined and courageous, exactly what their inspiration Hatton embodied.
