British light heavyweight Buatsi made a winning yet underwhelming start to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques failed to mesh, with excessive holding marring the contest.
The scoring judges scored it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my finest performance but what do champions do? They succeed," the victor said in the ring as he was faced negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, thirty-one, deserved the win but the British boxer suffered the second defeat of his 28-fight career.
The Champion Earns Win in Slow Affair
Buatsi's progress has plateaued in recent times through management issues and long spells of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to achieve expectations - maybe in what seems like a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers willing to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second bout at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a protective stance to edge the cleaner rounds.
The crowd's patience diminished amid frequent holding, with Parker going down various instances during close exchanges.
Parker closed well, but many rounds were hard to judge. Although, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - based on this performance - still appears challenging.
Hatton Memorial Fuels Manchester Derby Battle
In the supporting bout, Arthur beat Rea by close verdict in a outstanding local rivalry contest to secure the continental championship.
The stadium became quiet for a deeply poignant memorial before the fight as ten bells sounded in honor of local hero Hatton, who passed away 46 years old in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the final chime ended, the silence transitioned to a chant of "only one Hatton".
The competition during the bout reflected the feeling. The eventual winner was knocked down by a significant blow flush on the face in the second round, standing uncertainly to his feet on the eight count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was damaged quickly, heavy swelling appearing under his eye as the duo traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the action seldom decreased over twelve rounds.
The victor barely secured the decision with judges scorecards of draw, narrow lead and wider gap, but each competitor were relentless and brave, everything their inspiration the legend stood for.
