UK light-heavyweight Buatsi earned a win yet unimpressive start to life under new promoters with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight ten-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a match where the approaches didn't complement each other, with excessive holding marring the contest.
Two judges had it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my finest display but what defines champions? They prevail," Buatsi said in the ring as he was greeted with boos from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt the challenger, 31, deserved the win but the British boxer experienced the second loss of his 28-match professional journey.
Buatsi Claims Win in Uneventful Affair
The fighter's advancement has stalled in the past few years through promotional disputes and extended periods of lack of fights.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what feels like a last chance - under recent support.
He opened tentatively, with both competitors ready to be aggressive in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who operated with a protective stance to edge the more effective stanzas.
The crowd's patience decreased because of constant clinching, with Parker slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but many rounds were challenging to assess. Although, ex-champion Carl Frampton - working with Dazn - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by close verdict in a excellent Manchester vs Manchester contest to capture the continental title.
The arena grew hushed for a extremely emotional homage ahead of the contest as ten bells tolled in honor of Manchester's own Ricky Hatton, who succumbed aged 46 in recently.
It was the premier televised match card in the city since Hatton's death. As the final chime disappeared, the silence gave way to a chant of "there's only one Ricky Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was knocked down by a powerful punch squarely on the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was marked up soon, significant bruising forming under his face as the duo exchanged powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur edged the decision with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were persistent and full of heart, all the qualities their inspiration the legend represented.
