UK light-heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a judges' verdict in a close 10-round fight at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a fight where the approaches never quite clicked, with excessive holding disrupting the contest.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my greatest showing but what do champions do? They succeed," Buatsi said in the ringside as he was faced disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in February - to claim a 20th pro success.
Several boxing experts felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
Buatsi Claims Victory in Dull Match
The fighter's advancement has stalled in recent times through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He started carefully, with the two boxers ready to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, creating difficulties for Buatsi, who worked behind a protective stance to barely take the more effective rounds.
The crowd's patience diminished during constant clinching, with Parker going down various instances during close exchanges.
The underdog finished the stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Russian champion Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the supporting bout, Arthur beat Rea by divided decision in a terrific all-Manchester fight to secure the European light-heavyweight championship.
The arena fell silent for a deeply poignant memorial prior to the bout as ten bells rang in remembrance of local hero Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the last bell faded, the stillness became a shout of "only one Hatton".
The competition during the bout equaled the feeling. The eventual winner was floored by a left hook flush on the face in the early round, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, significant bruising appearing under his face as the duo exchanged hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur edged the result with judges scorecards of even score, 115-113 and clearer margin, but the two athletes were persistent and courageous, everything their inspiration Hatton represented.
