British light heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive ten-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a fight where the styles didn't complement each other, with excessive holding disrupting the contest.
Two judges scored it 96-94 for to Buatsi, with the remaining official a 95-95.
"It wasn't my finest display but how do champions respond? They win," Buatsi said in the arena as he was faced boos from the spectators.
The British fighter got back to winning after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the British boxer experienced the second setback of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Encounter
The fighter's advancement has stalled in recent years through management issues and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what appears to be a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors ready to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a protective stance to narrowly win the more effective rounds.
The spectators' acceptance diminished because of frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to secure the European light heavyweight championship.
The venue fell silent for a profoundly moving memorial before the fight as a bell ceremony tolled in memory of local hero Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime ended, the silence became a shout of "there's only one Hatton".
The action inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch squarely on the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the traditional red equipment of the fighter soon came forward again. From the other corner, Rea - in opposing blue - was damaged quickly, significant bruising developing under his orbital area as the fighters traded punishing blows.
The challenger, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
The victor barely secured the decision with scoring tallies of even score, 115-113 and clearer margin, but both fighters were persistent and full of heart, everything their idol Hatton embodied.
