British light-heavyweight Buatsi made a winning yet unimpressive debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a split decision in a competitive ten-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with constant grabbing disrupting the fight.
The officials scored it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my greatest performance but what do champions do? They prevail," Buatsi said in the arena as he was met by boos from the audience.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th career victory.
Various analysts felt the challenger, 31, merited the victory but the contender suffered the second defeat of his 28-match professional journey.
The Champion Claims Win in Uneventful Encounter
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He started carefully, with neither fighter willing to take risks in a tentative opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who utilized a protective stance to edge the more effective rounds.
The audience's tolerance diminished during regular grabbing, with Parker going down various instances during close exchanges.
The underdog closed well, but multiple periods were challenging to assess. Although, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just a single round.
Buatsi advances, but with the likes of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific local rivalry battle to secure the European light heavyweight title.
The arena grew hushed for a profoundly moving memorial prior to the bout as 10 bells tolled in honor of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the last bell disappeared, the silence transitioned to a roar of "only one Hatton".
The competition within the ring matched the emotion. Arthur was floored by a powerful punch directly to the jaw in the second round, rising shakily to his footing on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, Rea - in rival City blue - was marked up soon, noticeable damage developing under his face as the pair landed powerful shots.
Rea, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur edged the decision with judges scorecards of 114-114, close margin and clearer margin, but each competitor were persistent and full of heart, exactly what their inspiration Hatton stood for.
