UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But the challenger found success, delivering effective individual punches in a contest where the approaches didn't complement each other, with constant grabbing spoiling the contest.
Two judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my best display but what do champions do? They win," the victor said in the ringside as he was met by boos from the audience.
The Londoner bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Various analysts thought the challenger, 31, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-bout career.
Buatsi Earns Victory in Uneventful Affair
The fighter's advancement has slowed in recent times through contract problems and long spells of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to realize his potential - maybe in what feels like a final roll of the dice - under recent support.
He started carefully, with the two boxers ready to take risks in a tentative early stages.
Parker, whose sole prior loss came at the lower weight class against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second fight at light heavyweight.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who utilized a tight guard to edge the cleaner rounds.
The audience's tolerance wore thin because of frequent holding, with the challenger slipping to the canvas multiple occasions during grappling.
Parker closed well, but multiple periods were difficult to score. Although, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - judging by this showing - still looks steep.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific Manchester vs Manchester battle to win the European light-heavyweight championship.
The arena became quiet for a profoundly moving homage prior to the bout as a bell ceremony tolled in honor of Manchester's own Hatton, who died aged 46 in September.
It was the premier televised boxing event in the city since the passing of Hatton. As the final chime disappeared, the stillness became a shout of "there's only one Ricky Hatton".
The competition within the ring equaled the emotion. Arthur was floored by a significant blow directly to the face in the second round, rising shakily to his footing on the count of eight.
But the traditional red equipment of Arthur soon came forward again. From the other corner, the opponent - in rival City blue - was injured early, heavy swelling developing under his eye as the pair exchanged powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with stinging uppercuts as the pace seldom decreased over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were relentless and full of heart, everything their hero the legend represented.
