British light-heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive ten-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a fight where the styles didn't complement each other, with excessive holding marring the contest.
The scoring judges had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best display but what do champions do? They prevail," Buatsi commented in the arena as he was met by boos from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Various analysts believed the challenger, 31, merited the win but the British boxer suffered the second defeat of his 28-match career.
Buatsi Secures Triumph in Dull Affair
His career development has slowed in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He started carefully, with both competitors willing to take risks in a calculated opening few rounds.
The challenger, whose single earlier setback came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second appearance at the higher weight class.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who utilized a tight guard to narrowly win the cleaner stanzas.
The crowd's patience wore thin during constant clinching, with Parker slipping to the canvas multiple occasions during grappling.
The underdog finished the stronger, but many rounds were hard to judge. Although, former world champion Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just only one stanza.
Buatsi moves on, but with the presence of power puncher David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester fight to win the continental belt.
The venue became quiet for a deeply poignant memorial prior to the bout as ten bells rang in honor of the city's native Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound faded, the silence transitioned to a shout of "there's just one Ricky Hatton".
The competition inside the ropes matched the emotion. Arthur was floored by a left hook squarely on the face in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was damaged early, significant bruising developing under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the pace rarely dropped over twelve rounds.
The victor narrowly won the result with official scores of draw, 115-113 and clearer margin, but the two athletes were persistent and full of heart, exactly what their hero the legend represented.
