UK light heavyweight Buatsi earned a win yet underwhelming debut to life under fresh promotional backing with a decision win over Zach Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a split decision in a tight 10-round contest at the Manchester venue.
But Parker found success, delivering effective single shots in a fight where the styles never quite clicked, with constant grabbing spoiling the action.
The officials judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi commented in the arena as he was faced boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with his previous opponent in last winter - to earn a 20th career victory.
Some ringside observers thought the challenger, 31, should have been awarded the decision but the British boxer suffered the second loss of his 28-bout career.
Buatsi Claims Triumph in Uneventful Affair
The fighter's advancement has plateaued in recent times through contract problems and long spells of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with neither fighter prepared to take risks in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for Buatsi, who operated with a solid defense to edge the cleaner periods.
The spectators' acceptance diminished amid frequent holding, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi moves on, but with the caliber of power puncher Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Rea by split-decision in a terrific Manchester vs Manchester fight to win the continental championship.
The arena grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony rang in memory of the city's native Hatton, who died 46 years old in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound ended, the quiet transitioned to a chant of "there's just one Ricky Hatton".
The action within the ring equaled the emotion. Arthur was floored by a left hook flush on the jaw in the initial stage, getting up unsteadily to his position on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was marked up quickly, heavy swelling appearing under his face as the pair traded hard punches.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
Arthur barely secured the decision with official scores of even score, narrow lead and wider gap, but both fighters were persistent and courageous, everything their hero the legend represented.
