UK light-heavyweight Buatsi secured a victory yet underwhelming debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling single shots in a contest where the approaches failed to mesh, with frequent clinching marring the contest.
Two judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi said in the arena as he was met by boos from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts thought Parker, 31, merited the win but the Derbyshire fighter experienced the second setback of his 28-fight professional journey.
Buatsi Secures Win in Slow Affair
The fighter's advancement has plateaued in recent times through contract problems and considerable time of lack of fights.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a last chance - under new promotional backing.
He began cautiously, with the two boxers willing to take risks in a cagey early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who utilized a solid defense to barely take the better rounds.
The spectators' acceptance decreased because of regular grabbing, with the challenger falling to the floor several times during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Arthur overcame Bradley Rea by split-decision in a outstanding all-Manchester battle to capture the European light heavyweight championship.
The stadium grew hushed for a deeply poignant homage ahead of the contest as a bell ceremony sounded in honor of local hero Hatton, who died 46 years old in last month.
It was the first major televised match card in the location since Hatton's passing. As the last bell faded, the stillness gave way to a chant of "there's just one Hatton".
The fighting during the bout reflected the feeling. Arthur was sent to the canvas by a significant blow flush on the face in the initial stage, standing uncertainly to his feet on the eight count.
But the traditional red attire of Arthur soon came forward again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his eye as the fighters exchanged punishing blows.
Rea, 27, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor barely secured the verdict with scoring tallies of even score, 115-113 and wider gap, but each competitor were determined and courageous, all the qualities their hero Hatton embodied.
