British light heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a close 10-round bout at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a match where the styles failed to mesh, with constant grabbing spoiling the fight.
Two judges judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the ringside as he was greeted with disapproval from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed the challenger, thirty-one, merited the win but the British boxer endured the second defeat of his 28-fight career.
Buatsi Earns Win in Slow Affair
His career development has slowed in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - perhaps in what seems like a last chance - under fresh management.
He opened tentatively, with both competitors prepared to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a protective stance to narrowly win the cleaner stanzas.
The audience's tolerance diminished during regular grabbing, with the challenger going down multiple occasions during grappling.
The underdog finished the stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - providing analysis for Dazn - had it particularly wide for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the caliber of hard hitter Benavidez, and Russian star Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Fight
In the supporting bout, Arthur beat Bradley Rea by close verdict in a outstanding local rivalry contest to capture the European light-heavyweight title.
The venue fell silent for a deeply poignant tribute ahead of the contest as ten bells sounded in memory of Manchester's own Ricky Hatton, who died 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the last bell disappeared, the quiet became a roar of "there's only one Ricky Hatton".
The action during the bout matched the sentiment. Arthur was floored by a significant blow flush on the jaw in the early round, rising shakily to his feet on the eight count.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was injured quickly, noticeable damage appearing under his eye as the pair landed powerful shots.
Rea, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur edged the verdict with scoring tallies of 114-114, close margin and clearer margin, but both fighters were relentless and courageous, all the qualities their inspiration Hatton embodied.
