UK light heavyweight Buatsi made a winning yet underwhelming debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close ten-round contest at the Manchester venue.
But Parker found success, landing some telling individual punches in a match where the styles didn't complement each other, with frequent clinching spoiling the contest.
The officials had it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," Buatsi stated in the arena as he was greeted with negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an thrilling contest with Callum Smith in February - to claim a 20th pro success.
Several boxing experts felt Parker, 31, deserved the victory but the contender endured the second loss of his 28-match professional journey.
The Champion Secures Victory in Uneventful Encounter
The fighter's advancement has stalled in recent times through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of UK boxing, 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 ready to take risks in a calculated 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 making just his second bout at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a protective stance to narrowly win the cleaner rounds.
The audience's tolerance wore thin amid frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
The victor progresses, but with the likes of hard hitter Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Inspires All-Manchester Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester contest to secure the continental belt.
The arena grew hushed for a deeply poignant memorial before the fight as a bell ceremony sounded in honor of local hero Ricky Hatton, who died aged 46 in last month.
It was the first major televised boxing event in the location since Hatton's death. As the last bell disappeared, the silence became a roar of "only one Ricky Hatton".
The action inside the ropes equaled the feeling. Arthur was floored by a left hook flush on the jaw in the initial stage, standing uncertainly to his footing on the count of eight.
But the red-colored shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was injured quickly, significant bruising appearing under his face as the duo landed punishing blows.
Rea, 27, landed power punches but Arthur, 34, responded in kind with powerful upward punches as the action rarely dropped over twelve rounds.
Arthur barely secured the verdict with official scores of 114-114, close margin and 115-112, but both fighters were persistent and courageous, all the qualities their inspiration Hatton represented.
