British light heavyweight Buatsi made a winning yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a contest where the styles failed to mesh, with excessive holding spoiling the contest.
Two judges scored it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but what defines champions? They win," the victor said in the arena as he was faced negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, 31, should have been awarded the win but the Derbyshire fighter suffered the second defeat of his 28-bout career.
The Champion Earns Triumph in Dull Match
His career development has plateaued in recent years through contract problems and extended periods of inactivity.
Earlier forecast as the rising talent of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a last chance - under recent support.
He started carefully, with the two boxers prepared to take risks in a tentative initial rounds.
The challenger, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand mid-fight, was making just his second bout at light-heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective periods.
The crowd's patience diminished amid frequent holding, with Parker slipping to the canvas various instances during clinches.
Parker ended stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, awarding Buatsi just a single round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Hatton Homage Inspires All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight belt.
The venue became quiet for a deeply poignant memorial ahead of the contest as ten bells sounded in honor of local hero Hatton, who passed away 46 years old in recently.
It was the premier televised boxing event in the city since Hatton's death. As the last bell faded, the silence became a shout of "there's just one Ricky Hatton".
The action during the bout reflected the emotion. Arthur was knocked down by a powerful punch directly to the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged quickly, heavy swelling forming under his orbital area as the pair landed punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, matched the intensity with powerful upward punches as the intensity rarely dropped over the full distance.
The victor edged the decision with judges scorecards of 114-114, narrow lead and wider gap, but each competitor were persistent and brave, all the qualities their idol Hatton embodied.
