UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a split decision in a tight 10-round bout at the Manchester venue.
But Parker found success, landing some telling single shots in a fight where the approaches failed to mesh, with constant grabbing disrupting the fight.
The scoring judges scored it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," the victor commented in the arena as he was faced disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to secure a 20th pro success.
Several boxing experts believed the challenger, thirty-one, should have been awarded the victory but the contender suffered the second loss of his 28-fight professional journey.
Buatsi Claims Win in Uneventful Encounter
Buatsi's progress has stalled in recent times through management issues and considerable time of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under recent support.
He started carefully, with the two boxers ready to overcommit in a calculated initial rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who worked behind a solid defense to narrowly win the more effective periods.
The audience's tolerance wore thin during regular grabbing, with the challenger falling to the floor several times during grappling.
Parker finished the stronger, but multiple periods were hard to judge. Although, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Hatton Tribute Motivates All-Manchester Fight
In the secondary fight, Arthur beat Rea by close verdict in a excellent Manchester vs Manchester contest to capture the European light heavyweight championship.
The venue grew hushed for a deeply poignant tribute prior to the bout as ten bells sounded in remembrance of Manchester's own Hatton, who passed away 46 years old in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the concluding sound faded, the silence transitioned to a roar of "there's only one Ricky Hatton".
The action inside the ropes matched the sentiment. The eventual winner was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his position on the eight count.
But the traditional red attire of Arthur soon advanced again. Across the ring, the opponent - in rival City blue - was damaged quickly, significant bruising appearing under his orbital area as the pair exchanged powerful shots.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the intensity seldom decreased over the full distance.
The victor edged the decision with official scores of even score, 115-113 and wider gap, but both fighters were persistent and full of heart, everything their inspiration Hatton embodied.
