UK light-heavyweight Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Zach Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a majority decision in a tight 10-round fight at the boxing arena.
But the challenger found success, landing some telling clean blows in a contest where the techniques failed to mesh, with constant grabbing spoiling the fight.
The officials judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest display but how do champions respond? They win," the victor stated in the ringside as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th pro success.
Some ringside observers thought the challenger, thirty-one, should have been awarded the victory but the contender experienced the second setback of his 28-bout career.
The Champion Earns Win in Slow Affair
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now stands trying to achieve expectations - perhaps in what appears to be a last chance - under recent support.
He began cautiously, with both competitors willing to overcommit in a calculated early stages.
Parker, whose sole prior loss came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance wore thin amid constant clinching, with the challenger slipping to the canvas various instances during clinches.
Parker closed well, but several stanzas were hard to judge. Although, ex-champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Rea by divided decision in a outstanding local rivalry battle to capture the European light-heavyweight title.
The arena became quiet for a profoundly moving tribute ahead of the contest as a bell ceremony tolled in memory of local hero Ricky Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell ended, the stillness became a roar of "only one Ricky Hatton".
The competition during the bout matched the sentiment. Arthur was sent to the canvas by a significant blow flush on the chin in the early round, rising shakily to his feet on the count of eight.
But the red-colored shorts of Arthur soon came forward again. Across the ring, Rea - in competitive blue - was damaged soon, significant bruising developing under his face as the pair exchanged powerful shots.
The challenger, 27, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace hardly diminished over 12 rounds.
The victor narrowly won the decision with judges scorecards of 114-114, close margin and clearer margin, but both fighters were persistent and courageous, everything their inspiration Hatton embodied.
