British light heavyweight Buatsi made a winning yet disappointing debut to life under fresh promotional backing with a unanimous decision over Zach Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger had his moments, delivering effective individual punches in a contest where the styles didn't complement each other, with excessive holding spoiling the fight.
The officials had it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi commented in the arena as he was faced disapproval from the spectators.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought the challenger, 31, should have been awarded the decision but the contender suffered the second setback of his 28-bout career.
Buatsi Claims Victory in Slow Affair
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - possibly in what appears to be a final roll of the dice - under recent support.
He opened tentatively, with both competitors willing to take risks in a cagey opening few rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in two years ago after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who utilized a solid defense to edge the cleaner periods.
The crowd's patience wore thin amid frequent holding, with the challenger going down various instances during clinches.
The underdog ended stronger, but many rounds were hard to judge. However, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor progresses, but with the caliber of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Hatton Memorial Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester contest to capture the continental belt.
The venue fell silent for a extremely emotional memorial prior to the bout as a bell ceremony sounded in memory of the city's native Ricky Hatton, who died aged 46 in recently.
It was the first major televised boxing event in the city since Hatton's death. As the last bell ended, the quiet became a roar of "only one Ricky Hatton".
The competition within the ring reflected the emotion. The eventual winner was sent to the canvas by a left hook directly to the jaw in the initial stage, standing uncertainly to his feet on the count of eight.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was injured early, significant bruising appearing under his face as the duo traded powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over 12 rounds.
Arthur barely secured the result with scoring tallies of even score, narrow lead and 115-112, but each competitor were determined and courageous, all the qualities their hero Hatton embodied.
