UK light heavyweight Buatsi secured a victory yet unimpressive debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But Parker landed significant shots, delivering effective clean blows in a contest where the approaches didn't complement each other, with constant grabbing disrupting the contest.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best display but what do champions do? They prevail," Buatsi stated in the arena as he was greeted with disapproval from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Several boxing experts believed the challenger, 31, merited the win but the contender endured the second loss of his 28-bout professional journey.
Buatsi Earns Win in Dull Match
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of British boxing, he now stands trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers willing to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after suffering a hand injury in the contest, was participating in his second fight at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who worked behind a protective stance to barely take the cleaner rounds.
The spectators' acceptance decreased during constant clinching, with Parker going down various instances during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Homage Inspires All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding local rivalry fight to capture the European light-heavyweight title.
The arena became quiet for a profoundly moving homage prior to the bout as ten bells tolled in memory of local hero Hatton, who died at age 46 in September.
It was the first major televised match card in the city since Hatton's death. As the final chime faded, the quiet gave way to a roar of "only one Hatton".
The action inside the ropes matched the emotion. Arthur was floored by a left hook squarely on the face in the early round, rising shakily to his feet on the count of eight.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising developing under his face as the fighters landed punishing blows.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the action rarely dropped over the full distance.
The victor barely secured the verdict with judges scorecards of draw, close margin and 115-112, but the two athletes were persistent and brave, exactly what their inspiration Hatton stood for.
