British light-heavyweight Buatsi made a winning yet underwhelming start to life under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker found success, connecting with powerful clean blows in a match where the styles never quite clicked, with frequent clinching marring the action.
Two judges judged it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my finest performance but how do champions respond? They prevail," the victor commented in the arena as he was faced boos from the audience.
The Londoner bounced back after his maiden career setback - an epic battle with Callum Smith in February - to claim a 20th pro success.
Some ringside observers believed Parker, 31, deserved the victory but the contender endured the second setback of his 28-match career.
The Champion Claims Win in Uneventful Match
Buatsi's progress has plateaued in the past few years through management issues and extended periods of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second fight at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a tight guard to barely take the more effective stanzas.
The spectators' acceptance wore thin amid constant clinching, with Parker going down multiple occasions during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. Nevertheless, former world champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a terrific all-Manchester contest to capture the continental championship.
The venue fell silent for a extremely emotional memorial prior to the bout as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime disappeared, the quiet transitioned to a roar of "only one Ricky Hatton".
The competition inside the ropes reflected the emotion. Arthur was knocked down by a powerful punch directly to the face in the initial stage, rising shakily to his position on the count of eight.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in rival City blue - was injured quickly, noticeable damage forming under his face as the fighters exchanged punishing blows.
The challenger, 27, landed power punches but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur barely secured the result with official scores of even score, close margin and 115-112, but both fighters were determined and courageous, all the qualities their idol Hatton represented.
