UK light heavyweight Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a close 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling clean blows in a match where the styles never quite clicked, with constant grabbing spoiling the action.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but what defines champions? They succeed," the victor said in the ring as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts felt Parker, thirty-one, deserved the victory but the British boxer suffered the second loss of his 28-bout professional journey.
Buatsi Earns Triumph in Slow Match
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what appears to be a last chance - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to narrowly win the cleaner rounds.
The spectators' acceptance decreased during constant clinching, with Parker going down multiple occasions during clinches.
Parker closed well, but several stanzas were difficult to score. However, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the supporting bout, Arthur beat Bradley Rea by split-decision in a excellent all-Manchester battle to capture the European light-heavyweight championship.
The venue became quiet for a profoundly moving homage prior to the bout as ten bells sounded in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the concluding sound disappeared, the silence transitioned to a roar of "there's only one Ricky Hatton".
The fighting during the bout reflected the sentiment. The eventual winner was floored by a left hook directly to the chin in the second round, rising shakily to his position on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up soon, significant bruising appearing under his eye as the duo exchanged hard punches.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the decision with judges scorecards of draw, narrow lead and clearer margin, but each competitor were persistent and courageous, exactly what their hero Hatton stood for.
