UK light heavyweight Buatsi made a winning yet underwhelming beginning to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the approaches failed to mesh, with constant grabbing marring the fight.
The officials scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They win," the victor commented in the arena as he was faced negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts felt Parker, thirty-one, merited the decision but the Derbyshire fighter endured the second defeat of his 28-fight career.
The Champion Secures Triumph in Slow Affair
The fighter's advancement has plateaued in recent years through management issues and considerable time of lack of fights.
Once tipped as the next star of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with both competitors ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who utilized a protective stance to edge the better rounds.
The spectators' acceptance decreased during regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. However, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor progresses, but with the likes of power puncher Benavidez, and Eastern European fighter Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Motivates Local Rivalry Contest
In the co-main event, Arthur overcame Rea by split-decision in a terrific local rivalry fight to secure the European light-heavyweight championship.
The arena became quiet for a extremely emotional memorial ahead of the contest as ten bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in September.
It was the premier televised fight night in the city since Hatton's passing. As the final chime faded, the quiet transitioned to a shout of "there's just one Hatton".
The action within the ring matched the sentiment. Arthur was knocked down by a significant blow directly to the jaw in the early round, getting up unsteadily to his feet on the referee's count.
But the traditional red attire of Arthur soon pressed ahead again. Across the ring, Rea - in rival City blue - was injured quickly, noticeable damage forming under his eye as the fighters landed powerful shots.
Rea, twenty-seven, connected with strong blows but the champion, thirty-four, matched the intensity with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur edged the result with judges scorecards of even score, 115-113 and 115-112, but each competitor were determined and full of heart, all the qualities their idol the legend stood for.
