UK light heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a split decision in a tight ten-round contest at the boxing arena.
But the challenger had his moments, delivering effective clean blows in a fight where the styles failed to mesh, with constant grabbing marring the action.
The officials judged it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my best display but how do champions respond? They prevail," the victor said in the arena as he was faced negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers thought Parker, thirty-one, deserved the decision but the contender suffered the second setback of his 28-fight career.
The Champion Secures Victory in Slow Match
His career development has plateaued in the past few years through promotional disputes and extended periods of lack of fights.
Previously predicted as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under recent support.
He started carefully, with both competitors willing to take risks in a calculated initial rounds.
The challenger, whose sole prior loss 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 middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a solid defense to barely take the cleaner rounds.
The crowd's patience decreased because of frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
Parker closed well, but many rounds were hard to judge. Nevertheless, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Tribute Fuels Manchester Derby Contest
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to capture the European light-heavyweight championship.
The venue grew hushed for a deeply poignant tribute before the fight as ten bells rang in remembrance of local hero Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound disappeared, the quiet became a chant of "there's only one Hatton".
The fighting within the ring reflected the emotion. The eventual winner was knocked down by a significant blow squarely on the face in the second round, getting up unsteadily to his footing on the eight count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged soon, significant bruising developing under his orbital area as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over 12 rounds.
Arthur edged the result with judges scorecards of 114-114, close margin and 115-112, but each competitor were persistent and courageous, everything their inspiration Hatton represented.
