UK light-heavyweight Buatsi earned a win yet underwhelming start to his career under recent management with a unanimous decision over Zach Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close 10-round contest at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a contest where the styles never quite clicked, with excessive holding marring the contest.
Two judges judged it 96-94 to the winner, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They win," the victor said in the ring as he was faced disapproval from the audience.
The Londoner returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, merited the decision but the Derbyshire fighter endured the second defeat of his 28-bout career.
Buatsi Earns Win in Uneventful Encounter
Buatsi's progress has plateaued in recent years through promotional disputes and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now is positioned trying to achieve expectations - possibly in what appears to be a last chance - under new promotional backing.
He began cautiously, with both competitors ready to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the better periods.
The audience's tolerance decreased amid constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
Parker closed well, but many rounds were challenging to assess. Nevertheless, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to secure the continental belt.
The venue became quiet for a extremely emotional tribute prior to the bout as 10 bells tolled in remembrance of the city's native Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the last bell ended, the quiet gave way to a shout of "only one Hatton".
The competition during the bout equaled the emotion. Arthur was knocked down by a powerful punch squarely on the chin in the early round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged early, noticeable damage appearing under his eye as the fighters landed powerful shots.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur barely secured the result with judges scorecards of 114-114, 115-113 and clearer margin, but the two athletes were relentless and full of heart, everything their idol Hatton represented.
