UK light heavyweight Buatsi made a winning yet unimpressive beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a fight where the styles didn't complement each other, with excessive holding spoiling the fight.
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 win," the victor said in the arena as he was faced boos from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a 20th career victory.
Some ringside observers believed Parker, thirty-one, deserved the win but the contender suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Win in Slow Encounter
Buatsi's progress has plateaued in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to realize his potential - maybe in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors willing to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had kept it close, making life awkward for Buatsi, who utilized a protective stance to edge the better periods.
The crowd's patience diminished amid regular grabbing, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog closed well, but several stanzas were hard to judge. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it particularly wide for the challenger, awarding Buatsi just a single round.
Buatsi progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to secure the continental title.
The venue grew hushed for a profoundly moving homage prior to the bout as ten bells tolled in remembrance of local hero Hatton, who passed away at age 46 in September.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound faded, the silence transitioned to a shout of "only one Hatton".
The action inside the ropes reflected the feeling. Arthur was sent to the canvas by a significant blow squarely on the jaw in the early round, standing uncertainly to his position on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was marked up quickly, noticeable damage forming under his face as the duo landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the pace hardly diminished over twelve rounds.
Arthur narrowly won the result with judges scorecards of 114-114, narrow lead and wider gap, but the two athletes were relentless and brave, exactly what their inspiration the legend embodied.
