UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a contest where the styles never quite clicked, with frequent clinching disrupting the contest.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest performance but how do champions respond? They succeed," the victor said in the ring as he was greeted with boos from the audience.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to secure a 20th pro success.
Various analysts thought Parker, thirty-one, merited the win but the British boxer experienced the second loss of his 28-fight professional journey.
Buatsi Earns Triumph in Uneventful Encounter
Buatsi's progress has stalled in the past few years through management issues and considerable time of inactivity.
Previously predicted as the future of British boxing, he now stands trying to achieve expectations - possibly in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers prepared to take risks in a calculated initial rounds.
The challenger, whose sole prior loss came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, the underdog had stayed in contention, making life awkward for the favorite, who utilized a protective stance to barely take the more effective stanzas.
The spectators' acceptance diminished because of frequent holding, with Parker falling to the floor multiple occasions during clinches.
Parker closed well, but many rounds were hard to judge. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels Local Rivalry Contest
In the secondary fight, Arthur beat Rea by divided decision in a excellent all-Manchester battle to win the European light heavyweight title.
The venue became quiet for a profoundly moving homage ahead of the contest as a bell ceremony rang in memory of the city's native Hatton, who succumbed 46 years old in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the last bell ended, the quiet transitioned to a shout of "only one Ricky Hatton".
The fighting inside the ropes matched the sentiment. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored shorts of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was injured soon, significant bruising appearing under his face as the duo exchanged hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur barely secured the decision with official scores of even score, 115-113 and 115-112, but both fighters were relentless and courageous, exactly what their idol Hatton stood for.
