UK light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's punch output seemingly secured him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker had his moments, connecting with powerful clean blows in a match where the styles didn't complement each other, with frequent clinching disrupting the action.
The officials scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor commented in the arena as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts felt the challenger, thirty-one, merited the win but the Derbyshire fighter endured the second loss of his 28-fight professional journey.
The Champion Earns Win in Dull Match
His career development has plateaued in recent times through management issues and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter ready to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for Buatsi, who worked behind a tight guard to barely take the better rounds.
The audience's tolerance decreased because of regular grabbing, with Parker slipping to the canvas several times during close exchanges.
Parker finished the stronger, but several stanzas were difficult to score. However, previous titleholder the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the presence of hard hitter Benavidez, and Russian star Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the supporting bout, Arthur defeated Rea by divided decision in a terrific local rivalry battle to capture the continental title.
The venue grew hushed for a extremely emotional tribute prior to the bout as ten bells tolled in honor of local hero Ricky Hatton, who died 46 years old in last month.
It was the premier televised match card in the urban center since the passing of Hatton. As the last bell faded, the stillness gave way to a shout of "there's just one Ricky Hatton".
The action during the bout reflected the emotion. Arthur was knocked down by a significant blow flush on the face in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in opposing blue - was damaged early, noticeable damage forming under his eye as the duo landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace seldom decreased over the full distance.
The victor edged the result with judges scorecards of 114-114, 115-113 and clearer margin, but both fighters were relentless and courageous, all the qualities their inspiration the legend embodied.
