UK light-heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a contest where the techniques never quite clicked, with excessive holding spoiling the fight.
The scoring judges scored it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my finest showing but how do champions respond? They win," Buatsi stated in the ring as he was met by boos from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in February - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second loss of his 28-match professional journey.
The Champion Claims Triumph in Uneventful Affair
Buatsi's progress has stalled in recent times through promotional disputes and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a final roll of the dice - under fresh management.
He opened tentatively, with both competitors prepared to be aggressive in a cagey early stages.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who utilized a protective stance to barely take the more effective rounds.
The spectators' acceptance wore thin amid regular grabbing, with the challenger falling to the floor multiple occasions during grappling.
Parker closed well, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi moves on, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Hatton Homage Inspires All-Manchester Battle
In the supporting bout, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry contest to secure the European light heavyweight belt.
The arena grew hushed for a deeply poignant homage ahead of the contest as ten bells sounded in memory of local hero Hatton, who succumbed 46 years old in September.
It was the first major televised match card in the location since Hatton's passing. As the last bell disappeared, the silence transitioned to a shout of "only one Ricky Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was knocked down by a left hook directly to the face in the early round, getting up unsteadily to his footing on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling forming under his face as the pair landed punishing blows.
Rea, 27, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over the full distance.
Arthur edged the decision with judges scorecards of even score, narrow lead and 115-112, but the two athletes were determined and brave, all the qualities their idol Hatton embodied.
