UK light-heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a split decision in a tight ten-round fight at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a contest where the styles didn't complement each other, with frequent clinching spoiling the fight.
The scoring judges had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest performance but how do champions respond? They succeed," the victor said in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the win but the British boxer experienced the second loss of his 28-match career.
Buatsi Earns Win in Slow Match
His career development has slowed in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to achieve expectations - maybe in what appears to be a last chance - under recent support.
He started carefully, with the two boxers willing to overcommit in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to narrowly win the more effective rounds.
The spectators' acceptance decreased during frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were hard to judge. However, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Hatton Memorial Motivates Manchester Derby Fight
In the co-main event, Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the continental championship.
The stadium became quiet for a deeply poignant memorial prior to the bout as ten bells rang in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the first major televised fight night in the location since the passing of Hatton. As the concluding sound ended, the silence transitioned to a shout of "only one Ricky Hatton".
The competition inside the ropes equaled the sentiment. Arthur was knocked down by a powerful punch flush on the face in the early round, getting up unsteadily to his footing on the referee's count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was marked up soon, heavy swelling appearing under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, narrow lead and 115-112, but the two athletes were relentless and brave, all the qualities their inspiration the legend stood for.
