UK light-heavyweight Buatsi secured a victory yet underwhelming start to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a split decision in a tight 10-round contest at the boxing arena.
But Parker found success, landing some telling clean blows in a match where the approaches never quite clicked, with constant grabbing spoiling the action.
The officials scored it 96-94 to Buatsi, with the final judge a 95-95 draw.
"It wasn't my finest showing but what defines champions? They win," Buatsi said in the arena as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought the challenger, 31, deserved the win but the contender endured the second loss of his 28-bout career.
Buatsi Secures Win in Uneventful Match
His career development has plateaued in recent times through contract problems and long spells of lack of fights.
Previously predicted as the future of British boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers willing to be aggressive in a calculated initial rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand during the bout, was making just his second appearance at light heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who worked behind a solid defense to narrowly win the better rounds.
The audience's tolerance decreased amid constant clinching, with the challenger falling to the floor various instances during clinches.
The underdog ended stronger, but multiple periods were difficult to score. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor moves on, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Hatton Homage Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to capture the continental belt.
The venue became quiet for a profoundly moving tribute ahead of the contest as ten bells tolled in remembrance of local hero Ricky Hatton, who died 46 years old in recently.
It was the first major televised fight night in the urban center since the passing of Hatton. As the concluding sound ended, the silence gave way to a roar of "only one Hatton".
The action inside the ropes equaled the emotion. The eventual winner was sent to the canvas by a significant blow squarely on the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up soon, heavy swelling developing under his orbital area as the fighters traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the result with scoring tallies of 114-114, narrow lead and clearer margin, but each competitor were persistent and full of heart, everything their hero Hatton stood for.
