UK light-heavyweight Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a close ten-round fight at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a contest where the techniques didn't complement each other, with constant grabbing marring the contest.
Two judges scored it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They succeed," Buatsi commented in the ringside as he was met by disapproval from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with his previous opponent in February - to secure a 20th career victory.
Various analysts believed the challenger, 31, merited the decision but the British boxer experienced the second loss of his 28-bout professional journey.
Buatsi Secures Triumph in Uneventful Encounter
His career development has stalled in recent years through management issues and considerable time of lack of fights.
Earlier forecast as the next star of British boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a last chance - under fresh management.
He opened tentatively, with neither fighter willing to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the better periods.
The crowd's patience wore thin amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
Parker closed well, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor progresses, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Arthur beat Rea by close verdict in a terrific Manchester vs Manchester battle to win the continental championship.
The stadium fell silent for a deeply poignant tribute before the fight as ten bells rang in honor of local hero Hatton, who succumbed at age 46 in September.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the final chime disappeared, the silence gave way to a chant of "there's just one Ricky Hatton".
The competition during the bout matched the sentiment. The eventual winner was knocked down by a left hook flush on the face in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon advanced again. Across the ring, Rea - in competitive blue - was injured quickly, heavy swelling developing under his eye as the pair exchanged powerful shots.
The challenger, 27, delivered effective strikes but Arthur, 34, responded in kind with stinging uppercuts as the action seldom decreased over 12 rounds.
Arthur narrowly won the result with official scores of 114-114, 115-113 and 115-112, but the two athletes were determined and full of heart, exactly what their idol the legend embodied.
