UK light heavyweight Buatsi secured a victory yet disappointing beginning to life under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a tight 10-round bout at the boxing arena.
But Parker found success, landing some telling individual punches in a contest where the techniques didn't complement each other, with excessive holding marring the action.
The officials scored it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best showing but what defines champions? They prevail," Buatsi stated in the arena as he was faced negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts felt Parker, 31, merited the victory but the Derbyshire fighter experienced the second loss of his 28-match professional journey.
Buatsi Claims Triumph in Dull Match
His career development has slowed in recent years through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to overcommit in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had kept it close, making life awkward for the favorite, who operated with a protective stance to narrowly win the better stanzas.
The audience's tolerance decreased because of frequent holding, with Parker going down various instances during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor moves on, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to win the European light heavyweight championship.
The arena became quiet for a profoundly moving tribute prior to the bout as ten bells rang in memory of the city's native Ricky Hatton, who died 46 years old in last month.
It was the first major televised fight night in the city since Hatton's death. As the final chime disappeared, the stillness transitioned to a shout of "only one Hatton".
The competition during the bout reflected the emotion. Arthur was sent to the canvas by a significant blow directly to the jaw in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, the opponent - in competitive blue - was damaged early, significant bruising appearing under his eye as the pair landed hard punches.
The challenger, 27, landed power punches but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the decision with judges scorecards of even score, close margin and 115-112, but the two athletes were determined and full of heart, exactly what their hero Hatton stood for.
