UK light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a competitive 10-round bout at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a fight where the styles failed to mesh, with excessive holding disrupting the fight.
The scoring judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best showing but how do champions respond? They win," the victor said in the arena as he was greeted with negative reactions from the crowd.
The Londoner bounced back after his first professional defeat - an epic battle with his previous opponent in February - to earn a twentieth professional win.
Some ringside observers thought the challenger, 31, merited the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
Buatsi Claims Triumph in Dull Match
His career development has stalled in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - maybe in what appears to be a last chance - under recent support.
He opened tentatively, with neither fighter willing to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was making just his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a protective stance to edge the cleaner stanzas.
The spectators' acceptance wore thin amid constant clinching, with Parker going down multiple occasions during clinches.
Parker ended stronger, but multiple periods were difficult to score. However, former world champion Carl Frampton - commentating on Dazn - had it strongly leaning for Parker, awarding Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian champion Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Hatton Memorial Inspires Manchester Derby Battle
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a excellent Manchester vs Manchester contest to win the European light heavyweight title.
The stadium grew hushed for a profoundly moving tribute prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the initial significant broadcast fight night in the location since Hatton's death. As the final chime faded, the stillness became a shout of "there's just one Hatton".
The fighting inside the ropes reflected the feeling. Arthur was knocked down by a left hook squarely on the jaw in the initial stage, standing uncertainly to his footing on the referee's count.
But the traditional red equipment of Arthur soon advanced again. Across the ring, Rea - in rival City blue - was marked up early, significant bruising appearing under his orbital area as the pair traded hard punches.
The challenger, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
The victor narrowly won the result with scoring tallies of draw, narrow lead and 115-112, but each competitor were persistent and courageous, all the qualities their inspiration Hatton embodied.
