UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a close 10-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a contest where the approaches never quite clicked, with excessive holding spoiling the fight.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They win," Buatsi said in the ring as he was greeted with negative reactions from the crowd.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Various analysts thought the challenger, 31, deserved the victory but the contender suffered the second setback of his 28-bout career.
Buatsi Claims Win in Dull Match
His career development has slowed in recent years through contract problems and extended periods of lack of fights.
Earlier forecast as the future of UK boxing, he now stands trying to achieve expectations - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors willing to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second appearance at the higher weight class.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to edge the more effective periods.
The crowd's patience wore thin amid frequent holding, with the challenger falling to the floor various instances during grappling.
The underdog closed well, but several stanzas were difficult to score. However, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry fight to capture the European light-heavyweight championship.
The arena grew hushed for a profoundly moving tribute ahead of the contest as ten bells sounded in honor of local hero Ricky Hatton, who died aged 46 in September.
It was the premier televised fight night in the city since the passing of Hatton. As the last bell faded, the quiet became a roar of "there's only one Hatton".
The action within the ring matched the emotion. Arthur was knocked down by a significant blow flush on the jaw in the second round, rising shakily to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged early, heavy swelling developing under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over the full distance.
The victor edged the verdict with official scores of even score, 115-113 and wider gap, but each competitor were determined and courageous, everything their idol the legend represented.
