British light heavyweight Buatsi secured a victory yet unimpressive beginning to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a split decision in a close ten-round bout at the Manchester venue.
But the challenger had his moments, landing some telling single shots in a fight where the approaches failed to mesh, with constant grabbing disrupting the action.
The scoring judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my greatest display but how do champions respond? They succeed," Buatsi said in the ring as he was greeted with negative reactions from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought the challenger, thirty-one, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
The Champion Earns Triumph in Slow Affair
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to realize his potential - maybe in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with both competitors prepared to be aggressive in a cagey early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner stanzas.
The audience's tolerance diminished amid frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
The underdog closed well, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a outstanding all-Manchester fight to win the European light heavyweight championship.
The venue grew hushed for a deeply poignant tribute before the fight as a bell ceremony tolled in memory of Manchester's own Hatton, who died 46 years old in September.
It was the first major televised fight night in the city since Hatton's passing. As the last bell faded, the quiet gave way to a chant of "there's only one Hatton".
The action within the ring matched the feeling. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the early round, rising shakily to his footing on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was damaged soon, noticeable damage appearing under his eye as the duo traded powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity seldom decreased over the full distance.
The victor narrowly won the verdict with judges scorecards of even score, narrow lead and wider gap, but the two athletes were determined and courageous, exactly what their idol Hatton stood for.
