UK light heavyweight Buatsi secured a victory yet underwhelming debut to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, landing some telling individual punches in a fight where the techniques didn't complement each other, with frequent clinching spoiling the contest.
The officials scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my finest performance but what defines champions? They prevail," Buatsi said in the ringside as he was greeted with disapproval from the audience.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt the challenger, thirty-one, merited the win but the British boxer suffered the second setback of his 28-match career.
Buatsi Earns Triumph in Slow Match
The fighter's advancement has stalled in recent times through contract problems and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors willing to overcommit in a tentative initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was making just his second appearance at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who utilized a protective stance to barely take the more effective rounds.
The spectators' acceptance wore thin amid regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Fuels All-Manchester Contest
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a terrific local rivalry battle to secure the European light-heavyweight championship.
The venue grew hushed for a profoundly moving memorial ahead of the contest as 10 bells sounded in remembrance of Manchester's own Hatton, who died at age 46 in last month.
It was the first major televised match card in the location since the passing of Hatton. As the last bell ended, the quiet transitioned to a roar of "there's only one Ricky Hatton".
The competition inside the ropes equaled the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the initial stage, rising shakily to his feet on the referee's count.
But the red-colored attire of Arthur soon came forward again. From the other corner, Rea - in rival City blue - was marked up early, significant bruising developing under his eye as the fighters landed punishing blows.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur barely secured the verdict with official scores of draw, narrow lead and wider gap, but both fighters were determined and brave, all the qualities their inspiration the legend stood for.
