British light heavyweight Buatsi made a winning yet unimpressive start to his career under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful clean blows in a match where the approaches never quite clicked, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what do champions do? They prevail," Buatsi stated in the arena as he was greeted with disapproval from the audience.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought the challenger, 31, merited the victory but the Derbyshire fighter endured the second setback of his 28-fight career.
The Champion Secures Triumph in Uneventful Affair
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against his former opponent in two years ago after breaking his hand mid-fight, was making just his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to narrowly win the cleaner rounds.
The spectators' acceptance wore thin amid frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were difficult to score. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, giving Buatsi just one round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a excellent all-Manchester battle to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional tribute before the fight as a bell ceremony sounded in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the initial significant broadcast fight night in the city since Hatton's death. As the concluding sound faded, the quiet transitioned to a chant of "only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. Arthur was knocked down by a significant blow directly to the chin in the early round, rising shakily to his footing on the count of eight.
But the Manchester United-red equipment of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was marked up soon, noticeable damage forming under his face as the duo exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the intensity hardly diminished over twelve rounds.
The victor narrowly won the decision with judges scorecards of draw, 115-113 and clearer margin, but each competitor were relentless and full of heart, all the qualities their inspiration Hatton embodied.
