British light-heavyweight Buatsi secured a victory yet underwhelming debut to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger landed significant shots, delivering effective clean blows in a fight where the approaches never quite clicked, with frequent clinching marring the fight.
The scoring judges had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the ring as he was faced negative reactions from the audience.
The Londoner returned to winning ways after his first professional defeat - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Various analysts felt Parker, thirty-one, should have been awarded the decision but the British boxer experienced the second defeat of his 28-fight professional journey.
Buatsi Claims Victory in Slow Affair
His career development has plateaued in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors prepared to be aggressive in a cagey early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury during the bout, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to edge the better rounds.
The audience's tolerance diminished amid regular grabbing, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Homage Motivates All-Manchester Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific local rivalry contest to capture the European light heavyweight championship.
The venue grew hushed for a profoundly moving homage prior to the bout as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the location since the passing of Hatton. As the concluding sound faded, the stillness transitioned to a chant of "there's only one Hatton".
The action within the ring reflected the emotion. The eventual winner was knocked down by a significant blow directly to the jaw in the early round, rising shakily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured quickly, noticeable damage appearing under his face as the fighters landed powerful shots.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over the full distance.
The victor narrowly won the decision with scoring tallies of draw, 115-113 and wider gap, but the two athletes were relentless and full of heart, exactly what their hero Hatton represented.
