UK light-heavyweight Buatsi earned a win yet disappointing debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a majority decision in a competitive 10-round contest at the boxing arena.
But Parker found success, delivering effective single shots in a contest where the styles never quite clicked, with frequent clinching spoiling the fight.
The officials judged it 96-94 for to the winner, with the remaining official a 95-95.
"It wasn't my best showing but what do champions do? They win," Buatsi said in the ring as he was met by disapproval from the spectators.
The British fighter got back to winning after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Some ringside observers thought Parker, 31, merited the decision but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Secures Victory in Uneventful Encounter
The fighter's advancement has slowed in the past few years through management issues and extended periods of lack of fights.
Earlier forecast as the future of British boxing, he now is positioned trying to realize his potential - maybe in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter willing to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a solid defense to barely take the better stanzas.
The spectators' acceptance diminished during frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester battle to secure the European light heavyweight championship.
The stadium grew hushed for a extremely emotional homage prior to the bout as a bell ceremony sounded in honor of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound faded, the silence became a chant of "only one Ricky Hatton".
The competition during the bout equaled the feeling. The eventual winner was sent to the canvas by a left hook flush on the face in the early round, standing uncertainly to his position on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in opposing blue - was marked up early, heavy swelling appearing under his orbital area as the duo exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur narrowly won the result with judges scorecards of 114-114, narrow lead and clearer margin, but the two athletes were determined and courageous, everything their hero the legend stood for.
