UK light heavyweight Buatsi made a winning yet underwhelming start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a close 10-round bout at the Manchester venue.
But Parker had his moments, delivering effective single shots in a contest where the styles never quite clicked, with constant grabbing spoiling the fight.
Two judges scored it 96-94 to the winner, with the remaining official a even score.
"It wasn't my finest display but what defines champions? They win," the victor stated in the ringside as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to claim a 20th career victory.
Various analysts thought Parker, thirty-one, should have been awarded the decision but the Derbyshire fighter experienced the second defeat of his 28-fight career.
The Champion Earns Victory in Dull Affair
Buatsi's progress has plateaued in recent times through promotional disputes and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers willing to be aggressive in a calculated early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to narrowly win the cleaner rounds.
The crowd's patience decreased during constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but many rounds were difficult to score. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor progresses, but with the presence of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to win the continental championship.
The arena grew hushed for a deeply poignant memorial ahead of the contest as ten bells sounded in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the concluding sound faded, the quiet gave way to a chant of "there's only one Ricky Hatton".
The fighting within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow squarely on the face in the early round, standing uncertainly to his feet on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged early, noticeable damage appearing under his eye as the fighters landed hard punches.
Rea, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the verdict with scoring tallies of even score, close margin and wider gap, but both fighters were persistent and courageous, exactly what their inspiration Hatton stood for.
