UK light heavyweight Buatsi made a winning yet unimpressive debut to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a split decision in a close 10-round fight at the Manchester venue.
But Parker found success, landing some telling individual punches in a match where the approaches never quite clicked, with frequent clinching spoiling the action.
The officials judged it 96-94 to Buatsi, with the third a even score.
"It wasn't my finest display but what defines champions? They succeed," the victor said in the ringside as he was met by negative reactions from the spectators.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a twentieth professional win.
Various analysts thought Parker, thirty-one, should have been awarded the win but the contender experienced the second setback of his 28-match professional journey.
Buatsi Claims Victory in Dull Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under fresh management.
He started carefully, with neither fighter prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, causing problems for the favorite, who operated with a protective stance to barely take the better periods.
The spectators' acceptance diminished amid frequent holding, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent local rivalry contest to win the continental championship.
The venue fell silent for a profoundly moving homage prior to the bout as 10 bells sounded in honor of Manchester's own Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the concluding sound ended, the quiet became a shout of "there's just one Hatton".
The action within the ring equaled the feeling. The eventual winner was knocked down by a powerful punch directly to the chin in the initial stage, getting up unsteadily to his feet on the count of eight.
But the red-colored shorts of the fighter soon pressed ahead again. On the opposite side, Rea - in competitive blue - was injured soon, noticeable damage developing under his eye as the duo traded powerful shots.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur edged the result with official scores of 114-114, close margin and clearer margin, but each competitor were determined and courageous, everything their hero Hatton embodied.
