UK light-heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a contest where the approaches never quite clicked, with excessive holding marring the action.
Two judges had it 96-94 to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what defines champions? They succeed," Buatsi commented in the arena as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in February - to secure a twentieth professional win.
Some ringside observers felt Parker, thirty-one, should have been awarded the decision but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Earns Victory in Uneventful Encounter
His career development has slowed in recent times through promotional disputes and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to fulfil that early promise - possibly in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers willing to overcommit in a calculated early stages.
Parker, whose sole prior loss came at the lower weight class against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who operated with a protective stance to narrowly win the more effective rounds.
The crowd's patience wore thin because of regular grabbing, with Parker falling to the floor several times during grappling.
The underdog closed well, but multiple periods were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian champion Bivol ruling the division, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Fight
In the supporting bout, Arthur beat Rea by divided decision in a excellent local rivalry fight to secure the European light-heavyweight title.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony rang in remembrance of the city's native Ricky Hatton, who died aged 46 in last month.
It was the first major televised match card in the urban center since Hatton's death. As the last bell disappeared, the quiet became a roar of "there's just one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the eight count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was marked up quickly, noticeable damage forming under his eye as the fighters landed hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, matched the intensity with effective uppercuts as the intensity seldom decreased over 12 rounds.
The victor barely secured the result with scoring tallies of draw, 115-113 and 115-112, but the two athletes were relentless and full of heart, exactly what their inspiration the legend represented.
