British light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a close 10-round fight at the Manchester venue.
But the challenger found success, delivering effective clean blows in a contest where the techniques never quite clicked, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to Buatsi, with the final judge a even score.
"It wasn't my best display but what defines champions? They prevail," the victor commented in the ring as he was faced negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a 20th career victory.
Various analysts felt the challenger, 31, merited the win but the Derbyshire fighter endured the second setback of his 28-match career.
Buatsi Claims Victory in Uneventful Match
His career development has slowed in the past few years through contract problems and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers 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 suffering a hand injury in the contest, was making just his second bout at light-heavyweight.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who operated with a protective stance to edge the better periods.
The crowd's patience wore thin because of regular grabbing, with Parker going down multiple occasions during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Although, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Motivates Manchester Derby Battle
In the co-main event, Arthur defeated Rea by divided decision in a terrific all-Manchester contest to capture the European light heavyweight belt.
The venue became quiet for a deeply poignant homage ahead of the contest as a bell ceremony tolled in honor of Manchester's own Ricky Hatton, who died at age 46 in September.
It was the first major televised match card in the urban center since Hatton's death. As the last bell ended, the quiet transitioned to a roar of "there's just one Ricky Hatton".
The action inside the ropes matched the feeling. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, the opponent - in rival City blue - was marked up quickly, heavy swelling forming under his face as the duo landed powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over the full distance.
The victor barely secured the verdict with official scores of 114-114, close margin and 115-112, but the two athletes were determined and brave, all the qualities their hero the legend represented.
