UK light-heavyweight Buatsi made a winning yet unimpressive start to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive 10-round contest at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches never quite clicked, with constant grabbing disrupting the contest.
Two judges scored it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They prevail," Buatsi said in the ringside as he was greeted with boos from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to earn a twentieth professional win.
Some ringside observers believed the challenger, thirty-one, deserved the decision but the Derbyshire fighter endured the second defeat of his 28-bout career.
Buatsi Claims Triumph in Slow Affair
The fighter's advancement has stalled in recent times through management issues and long spells of inactivity.
Earlier forecast as the rising talent of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under fresh management.
He began cautiously, with the two boxers prepared to overcommit in a calculated initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who utilized a tight guard to barely take the cleaner periods.
The audience's tolerance decreased amid frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just one round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Russian champion Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the supporting bout, Arthur defeated Rea by split-decision in a terrific Manchester vs Manchester contest to capture the continental belt.
The stadium fell silent for a deeply poignant memorial prior to the bout as 10 bells tolled in memory of Manchester's own Hatton, who succumbed 46 years old 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 silence transitioned to a chant of "there's just one Ricky Hatton".
The action inside the ropes matched the feeling. Arthur was knocked down by a left hook squarely on the face in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up quickly, significant bruising appearing under his orbital area as the duo exchanged punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over 12 rounds.
The victor barely secured the decision with scoring tallies of draw, close margin and wider gap, but both fighters were relentless and courageous, everything their hero the legend stood for.
