UK light heavyweight Buatsi made a winning yet underwhelming beginning to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective single shots in a match where the approaches failed to mesh, with excessive holding spoiling the action.
Two judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my best display but what do champions do? They prevail," the victor commented in the arena as he was greeted with disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th career victory.
Various analysts felt the challenger, 31, should have been awarded the decision but the British boxer suffered the second setback of his 28-match career.
Buatsi Claims Win in Uneventful Encounter
His career development has stalled in the past few years through promotional disputes and considerable time of lack of fights.
Earlier forecast as the rising talent of British boxing, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under fresh management.
He started carefully, with both competitors prepared to take risks in a cagey opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who utilized a solid defense to barely take the better stanzas.
The crowd's patience decreased during frequent holding, with Parker going down several times during grappling.
Parker finished the stronger, but multiple periods were hard to judge. However, former world champion the boxing analyst - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol ruling the division, the path to championship success - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester battle to win the European light-heavyweight title.
The arena fell silent for a extremely emotional tribute prior to the bout as a bell ceremony rang in honor of local hero Ricky Hatton, who passed away at age 46 in last month.
It was the premier televised boxing event in the city since Hatton's passing. As the final chime faded, the silence gave way to a shout of "only one Ricky Hatton".
The fighting inside the ropes matched the emotion. Arthur was knocked down by a left hook squarely on the chin in the second round, rising shakily to his feet on the eight count.
But the red-colored shorts of the fighter soon came forward again. From the other corner, Rea - in rival City blue - was marked up soon, heavy swelling developing under his face as the duo exchanged punishing blows.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur edged the decision with judges scorecards of draw, 115-113 and 115-112, but each competitor were determined and courageous, all the qualities their inspiration the legend stood for.
