British light-heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive ten-round contest at the Manchester venue.
But the challenger had his moments, delivering effective clean blows in a fight where the approaches failed to mesh, with frequent clinching disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my finest performance but what defines champions? They win," the victor said in the ringside as he was greeted with boos from the audience.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with Callum Smith in February - to secure a 20th career victory.
Various analysts believed Parker, 31, should have been awarded the decision but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
Buatsi Secures Win in Slow Match
Buatsi's progress has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what seems like a last chance - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after damaging his hand during the bout, was participating in his second fight at light-heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the better stanzas.
The spectators' acceptance wore thin amid regular grabbing, with Parker falling to the floor several times during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Fuels Local Rivalry Battle
In the co-main event, Arthur overcame Bradley Rea by close verdict in a excellent local rivalry fight to capture the European light-heavyweight belt.
The stadium fell silent for a extremely emotional memorial ahead of the contest as 10 bells tolled in honor of Manchester's own Hatton, who died aged 46 in September.
It was the first major televised fight night in the city since Hatton's passing. As the concluding sound ended, the silence transitioned to a chant of "only one Hatton".
The action during the bout reflected the feeling. The eventual winner was sent to the canvas by a significant blow directly to the face in the initial stage, standing uncertainly to his footing on the eight count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured early, noticeable damage appearing under his face as the duo traded punishing blows.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur narrowly won the decision with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were persistent and full of heart, all the qualities their inspiration Hatton stood for.
