British light heavyweight Buatsi earned a win yet disappointing start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a close ten-round bout at the boxing arena.
But Parker found success, delivering effective individual punches in a match where the styles didn't complement each other, with excessive holding marring the contest.
The officials judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my finest performance but how do champions respond? They prevail," the victor stated in the ring as he was met by negative reactions from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to claim a 20th career victory.
Several boxing experts felt Parker, 31, merited the win but the British boxer endured the second setback of his 28-bout career.
The Champion Secures Victory in Dull Encounter
The fighter's advancement has slowed in the past few years through management issues and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a last chance - under recent support.
He began cautiously, with both competitors ready to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who operated with a tight guard to narrowly win the more effective stanzas.
The crowd's patience wore thin during regular grabbing, with Parker falling to the floor several times during clinches.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just one round.
Buatsi progresses, but with the presence of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still seems difficult.
Hatton Homage Fuels Manchester Derby Contest
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester contest to capture the European light-heavyweight belt.
The stadium fell silent for a profoundly moving tribute prior to the bout as ten bells sounded in memory of local hero Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the city since the passing of Hatton. As the last bell faded, the silence gave way to a chant of "there's only one Hatton".
The action during the bout reflected the feeling. Arthur was floored by a left hook directly to the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was damaged early, heavy swelling appearing under his eye as the duo exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the decision with scoring tallies of 114-114, 115-113 and wider gap, but both fighters were relentless and brave, all the qualities their hero Hatton stood for.
