UK light-heavyweight Buatsi secured a victory yet disappointing start to his career under fresh promotional backing with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a majority decision in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, connecting with powerful single shots in a match where the styles failed to mesh, with excessive holding spoiling the contest.
The scoring judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," the victor said in the ring as he was met by disapproval from the audience.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers felt Parker, 31, deserved the victory but the Derbyshire fighter endured the second setback of his 28-fight career.
The Champion Claims Triumph in Uneventful Encounter
The fighter's advancement has plateaued in recent years through management issues and long spells of lack of fights.
Earlier forecast as the future of British boxing, he now stands trying to realize his potential - perhaps in what seems like a last chance - under fresh management.
He started carefully, with the two boxers willing to be aggressive in a cagey early stages.
Parker, whose single earlier setback 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 halfway stage, Parker had made it competitive, creating difficulties for the favorite, who worked behind a protective stance to edge the better rounds.
The audience's tolerance wore thin amid regular grabbing, with Parker going down multiple occasions during grappling.
Parker ended stronger, but many rounds were challenging to assess. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Fuels Local Rivalry Contest
In the secondary fight, Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester battle to win the European light heavyweight title.
The venue grew hushed for a profoundly moving memorial prior to the bout as 10 bells sounded in memory of the city's native Hatton, who passed away at age 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the final chime ended, the quiet gave way to a roar of "there's just one Ricky Hatton".
The fighting during the bout matched the feeling. The eventual winner was knocked down by a powerful punch flush on the jaw in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was injured early, noticeable damage developing under his face as the duo landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over the full distance.
Arthur barely secured the verdict with judges scorecards of draw, narrow lead and wider gap, but the two athletes were determined and courageous, all the qualities their idol Hatton embodied.
