British light heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a tight ten-round bout at the Manchester venue.
But Parker landed significant shots, landing some telling individual punches in a fight where the approaches didn't complement each other, with excessive holding marring the fight.
Two judges had it 96-94 in favor to Buatsi, with the final judge a even score.
"It wasn't my finest showing but what do champions do? They win," the victor said in the arena as he was faced negative reactions from the audience.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts thought Parker, 31, deserved the win but the contender endured the second defeat of his 28-fight professional journey.
Buatsi Claims Triumph in Slow Match
His career development has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now stands trying to realize his potential - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with neither fighter prepared to overcommit in a calculated early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who worked behind a solid defense to narrowly win the more effective periods.
The audience's tolerance wore thin amid constant clinching, with Parker going down several times during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, former world champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, awarding Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian champion Bivol leading the category, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding all-Manchester battle to capture the continental championship.
The venue grew hushed for a profoundly moving memorial ahead of the contest as a bell ceremony rang in remembrance of the city's native Hatton, who passed away aged 46 in last month.
It was the first major televised match card in the city since Hatton's passing. As the last bell ended, the silence transitioned to a shout of "only one Ricky Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the jaw in the early round, rising shakily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up early, heavy swelling developing under his eye as the fighters traded punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the pace rarely dropped over twelve rounds.
Arthur edged the result with scoring tallies of 114-114, narrow lead and 115-112, but both fighters were relentless and brave, all the qualities their hero Hatton stood for.
