British light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive 10-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling individual punches in a match where the approaches never quite clicked, with excessive holding marring the contest.
The scoring judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They win," the victor stated in the ring as he was met by boos from the audience.
The Londoner got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Various analysts believed Parker, thirty-one, merited the win but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
The Champion Earns Triumph in Slow Encounter
Buatsi's progress has plateaued in the past few years through management issues and long spells of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to be aggressive in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was making just his second bout at light-heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to edge the better stanzas.
The audience's tolerance diminished amid constant clinching, with the challenger slipping to the canvas several times during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the secondary fight, Arthur overcame Bradley Rea by split-decision in a terrific local rivalry contest to win the European light-heavyweight championship.
The venue grew hushed for a profoundly moving memorial before the fight as 10 bells rang in memory of local hero Ricky Hatton, who passed away 46 years old in September.
It was the first major televised fight night in the city since the passing of Hatton. As the last bell ended, the silence became a shout of "only one Ricky Hatton".
The fighting within the ring matched the feeling. The eventual winner was sent to the canvas 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 attire of the fighter soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up early, heavy swelling developing under his orbital area as the duo landed powerful shots.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
Arthur edged the decision with judges scorecards of 114-114, narrow lead and wider gap, but each competitor were persistent and full of heart, exactly what their inspiration Hatton stood for.
