UK light-heavyweight Buatsi made a winning yet underwhelming beginning to life under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger found success, landing some telling single shots in a contest where the techniques never quite clicked, with constant grabbing marring the fight.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest performance but what defines champions? They prevail," the victor said in the arena as he was met by disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts felt the challenger, 31, should have been awarded the win but the Derbyshire fighter endured the second setback of his 28-match career.
The Champion Earns Victory in Uneventful Encounter
His career development has plateaued in recent years through promotional disputes and extended periods of lack of fights.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who utilized a tight guard to narrowly win the cleaner rounds.
The audience's tolerance diminished amid frequent holding, with Parker slipping to the canvas various instances during grappling.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of heavy hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Tribute Inspires Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding local rivalry battle to secure the European light heavyweight title.
The stadium fell silent for a deeply poignant memorial before the fight as ten bells rang in memory of the city's native Hatton, who passed away 46 years old in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound disappeared, the silence gave way to a roar of "there's only one Hatton".
The action within the ring equaled the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the second round, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, Rea - in opposing blue - was damaged quickly, significant bruising appearing under his orbital area as the duo traded punishing blows.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the action hardly diminished over the full distance.
Arthur barely secured the verdict with official scores of draw, 115-113 and 115-112, but the two athletes were determined and brave, exactly what their idol the legend stood for.
