British light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a points victory over Zach Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the techniques didn't complement each other, with constant grabbing spoiling the action.
The officials scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They win," the victor said in the ring as he was met by disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Various analysts thought the challenger, 31, deserved the victory but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
Buatsi Earns Win in Slow Encounter
The fighter's advancement has stalled in recent times through management issues and considerable time of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers ready to take risks in a tentative early stages.
The challenger, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second fight at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the more effective rounds.
The crowd's patience wore thin amid constant clinching, with Parker slipping to the canvas several times during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the caliber of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Arthur overcame Rea by split-decision in a outstanding local rivalry fight to win the European light-heavyweight championship.
The stadium became quiet for a deeply poignant homage ahead of the contest as 10 bells rang in remembrance of the city's native Hatton, who died aged 46 in last month.
It was the first major televised boxing event in the city since the passing of Hatton. As the concluding sound disappeared, the quiet transitioned to a chant of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. The eventual winner was knocked down by a powerful punch directly to the face in the initial stage, rising shakily to his feet on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged quickly, significant bruising forming under his orbital area as the duo landed hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with powerful upward punches as the action hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of draw, close margin and clearer margin, but each competitor were relentless and brave, everything their hero Hatton represented.
