UK light-heavyweight Buatsi made a winning yet disappointing debut 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 majority decision in a competitive 10-round contest at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a contest where the approaches never quite clicked, with frequent clinching spoiling the contest.
Two judges judged it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my greatest display but what defines champions? They prevail," the victor said in the ring as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts thought Parker, 31, deserved the victory but the British boxer endured the second setback of his 28-match professional journey.
The Champion Secures Triumph in Dull Match
Buatsi's progress has plateaued in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who utilized a tight guard to narrowly win the more effective rounds.
The crowd's patience decreased because of frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester contest to win the European light heavyweight title.
The venue grew hushed for a profoundly moving tribute before the fight as 10 bells sounded in honor of local hero Ricky Hatton, who died aged 46 in recently.
It was the premier televised match card in the location since Hatton's passing. As the final chime faded, the stillness became a shout of "there's only one Hatton".
The fighting inside the ropes equaled the feeling. Arthur was knocked down by a left hook flush on the face in the second round, rising shakily to his footing on the referee's count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was damaged soon, heavy swelling forming under his face as the fighters exchanged powerful shots.
Rea, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur narrowly won the decision with official scores of even score, close margin and clearer margin, but the two athletes were persistent and courageous, exactly what their inspiration Hatton represented.
