British light-heavyweight Buatsi earned a win yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a match where the styles never quite clicked, with frequent clinching disrupting the contest.
The scoring judges judged it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my greatest performance but how do champions respond? They prevail," the victor said in the arena as he was met by negative reactions from the audience.
The British fighter bounced back after his maiden career setback - an thrilling contest with his previous opponent in last winter - to claim a 20th pro success.
Several boxing experts believed Parker, 31, deserved the victory but the Derbyshire fighter endured the second setback of his 28-fight professional journey.
The Champion Secures Win in Dull Affair
Buatsi's progress has slowed in the past few years through contract problems and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a last chance - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a cagey opening few rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who utilized a solid defense to barely take the better periods.
The spectators' acceptance wore thin amid regular grabbing, with Parker slipping to the canvas various instances during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
The victor advances, but with the presence of heavy hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific local rivalry fight to win the European light heavyweight title.
The venue grew hushed for a deeply poignant tribute ahead of the contest as ten bells rang in remembrance of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound faded, the silence transitioned to a roar of "there's only one Hatton".
The competition inside the ropes reflected the emotion. Arthur was floored by a powerful punch directly to the face in the second round, standing uncertainly to his footing on the eight count.
But the red-colored equipment of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was damaged early, heavy swelling appearing under his orbital area as the duo exchanged hard punches.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with effective uppercuts as the intensity seldom decreased over the full distance.
Arthur barely secured the result with official scores of 114-114, close margin and wider gap, but each competitor were persistent and courageous, all the qualities their idol Hatton stood for.
