British light-heavyweight Buatsi earned a win yet unimpressive start to life under fresh promotional backing with a decision win over Zach Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive 10-round contest at the boxing arena.
But Parker found success, connecting with powerful clean blows in a fight where the styles didn't complement each other, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to the winner, with the third a even score.
"It wasn't my finest display but how do champions respond? They succeed," the victor stated in the arena as he was faced boos from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts felt Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second setback of his 28-match career.
The Champion Claims Triumph in Dull Encounter
Buatsi's progress has stalled in the past few years through management issues and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to take risks in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a solid defense to barely take the cleaner rounds.
The audience's tolerance wore thin during frequent holding, with Parker going down several times during clinches.
Parker closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
The victor progresses, but with the presence of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Inspires All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a excellent local rivalry contest to secure the European light heavyweight belt.
The venue grew hushed for a extremely emotional memorial before the fight as ten bells rang in honor of local hero Ricky Hatton, who succumbed 46 years old in last month.
It was the first major televised match card in the urban center since Hatton's death. As the concluding sound faded, the silence became a shout of "only one Hatton".
The action inside the ropes matched the sentiment. Arthur was sent to the canvas by a significant blow squarely on the jaw in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up soon, heavy swelling developing under his eye as the fighters exchanged hard punches.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
The victor barely secured the decision with scoring tallies of draw, narrow lead and wider gap, but both fighters were relentless and courageous, everything their inspiration the legend represented.
