UK light heavyweight Buatsi made a winning yet disappointing beginning 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 judges' verdict in a competitive ten-round fight at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the approaches failed to mesh, with excessive holding spoiling the fight.
The officials judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest display but what defines champions? They win," Buatsi said in the arena as he was greeted with boos from the spectators.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts believed Parker, 31, merited the victory but the Derbyshire fighter suffered the second defeat of his 28-bout career.
The Champion Earns Victory in Uneventful Encounter
The fighter's advancement has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what seems like a ultimate opportunity - under new promotional backing.
He began cautiously, with both competitors ready to take risks in a tentative early stages.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who utilized a protective stance to edge the better rounds.
The audience's tolerance wore thin amid regular grabbing, with Parker slipping to the canvas various instances during clinches.
Parker closed well, but multiple periods were difficult to score. However, previous titleholder Carl Frampton - commentating on the broadcast network - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Inspires Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a excellent local rivalry contest to win the European light heavyweight belt.
The arena fell silent for a extremely emotional homage prior to the bout as a bell ceremony sounded in remembrance of local hero Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the last bell ended, the stillness became a shout of "only one Ricky Hatton".
The action within the ring reflected the sentiment. The eventual winner was knocked down by a left hook flush on the chin in the initial stage, rising shakily to his feet on the count of eight.
But the red-colored equipment of Arthur soon advanced again. From the other corner, the opponent - in opposing blue - was injured soon, significant bruising forming under his orbital area as the fighters exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of draw, narrow lead and clearer margin, but the two athletes were determined and brave, all the qualities their hero Hatton embodied.
