UK light-heavyweight Buatsi made a winning yet underwhelming start to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive 10-round bout at the boxing arena.
But Parker had his moments, delivering effective single shots in a match where the styles failed to mesh, with frequent clinching marring the contest.
Two judges had it 96-94 in favor to Buatsi, with the third a 95-95 draw.
"It wasn't my best showing but what do champions do? They win," the victor stated in the ring as he was met by disapproval from the crowd.
The British fighter bounced back after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th pro success.
Several boxing experts felt the challenger, thirty-one, merited the victory but the British boxer experienced the second loss of his 28-bout career.
Buatsi Claims Victory in Uneventful Affair
Buatsi's progress has slowed in recent times through management issues and long spells of inactivity.
Earlier forecast as the future of British boxing, he now is positioned trying to achieve expectations - maybe in what feels like a last chance - under recent support.
He began cautiously, with neither fighter prepared to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who operated with a protective stance to barely take the better periods.
The audience's tolerance wore thin during regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the likes of hard hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester battle to win the European light-heavyweight belt.
The stadium fell silent for a deeply poignant memorial ahead of the contest as a bell ceremony tolled in remembrance of local hero Ricky Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's death. As the last bell ended, the silence became a roar of "only one Ricky Hatton".
The action inside the ropes matched the emotion. Arthur was floored by a powerful punch flush on the chin in the early round, getting up unsteadily to his position on the eight count.
But the red-colored equipment of the fighter soon advanced again. From the other corner, Rea - in rival City blue - was damaged soon, significant bruising developing under his face as the fighters traded hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with powerful upward punches as the intensity hardly diminished over 12 rounds.
The victor narrowly won the result with official scores of even score, close margin and 115-112, but both fighters were relentless and brave, exactly what their hero Hatton represented.
