UK light heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a majority decision in a close ten-round fight at the Manchester venue.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques failed to mesh, with frequent clinching spoiling the action.
The scoring judges had it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They win," Buatsi commented in the ring as he was greeted with boos from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to earn a 20th career victory.
Several boxing experts believed Parker, thirty-one, should have been awarded the win but the British boxer suffered the second defeat of his 28-match professional journey.
Buatsi Secures Victory in Dull Match
His career development has stalled in recent years through management issues and extended periods of inactivity.
Previously predicted as the next star of British boxing, he now finds himself trying to realize his potential - possibly in what feels like a last chance - under new promotional backing.
He started carefully, with neither fighter willing to be aggressive in a calculated opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who worked behind a protective stance to edge the more effective stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger going down several times during grappling.
Parker ended stronger, but many rounds were challenging to assess. Although, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the likes of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Memorial Motivates Manchester Derby Fight
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a excellent Manchester vs Manchester battle to win the European light heavyweight title.
The stadium fell silent for a extremely emotional memorial ahead of the contest as 10 bells sounded in remembrance of the city's native Hatton, who died aged 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the final chime disappeared, the stillness gave way to a roar of "there's only one Hatton".
The fighting within the ring equaled the emotion. Arthur was floored by a significant blow directly to the face in the early round, rising shakily to his footing on the eight count.
But the traditional red attire of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was damaged soon, noticeable damage developing under his eye as the fighters traded hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over 12 rounds.
Arthur narrowly won the verdict with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were persistent and full of heart, all the qualities their inspiration Hatton stood for.
