British light heavyweight Buatsi earned a win yet disappointing debut to life under fresh promotional backing with a points victory over Zach Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a contest where the styles didn't complement each other, with excessive holding marring the fight.
Two judges judged it 96-94 to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They win," Buatsi said in the ring as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-fight professional journey.
Buatsi Claims Victory in Dull Match
His career development has slowed in recent times through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with both competitors prepared to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for Buatsi, who operated with a protective stance to barely take the better periods.
The crowd's patience diminished amid constant clinching, with the challenger slipping to the canvas various instances during grappling.
Parker finished the stronger, but several stanzas were difficult to score. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
Buatsi moves on, but with the likes of heavy hitter Benavidez, and Russian star Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the European light heavyweight belt.
The stadium grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony rang in honor of local hero Ricky Hatton, who died at age 46 in recently.
It was the first major televised fight night in the location since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a chant of "only one Hatton".
The fighting inside the ropes reflected the sentiment. Arthur was knocked down by a significant blow squarely on the jaw in the second round, rising shakily to his feet on the count of eight.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged quickly, heavy swelling developing under his face as the duo exchanged punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor narrowly won the decision with official scores of draw, narrow lead and clearer margin, but both fighters were determined and full of heart, everything their inspiration Hatton represented.
