British light heavyweight Buatsi made a winning yet unimpressive start to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a judges' verdict in a close ten-round fight at the Manchester venue.
But the challenger found success, landing some telling single shots in a contest where the techniques failed to mesh, with frequent clinching spoiling the action.
Two judges had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best showing but what do champions do? They prevail," Buatsi said in the ring as he was greeted with disapproval from the spectators.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts believed Parker, thirty-one, merited the win but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
The Champion Claims Win in Uneventful Match
Buatsi's progress has slowed in recent times through management issues and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now finds himself trying to realize his potential - possibly in what appears to be a last chance - under new promotional backing.
He opened tentatively, with both competitors ready to be aggressive in a calculated initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to barely take the cleaner rounds.
The audience's tolerance wore thin because of constant clinching, with Parker going down multiple occasions during clinches.
The underdog closed well, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Memorial Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester fight to win the European light heavyweight championship.
The arena fell silent for a profoundly moving tribute ahead of the contest as ten bells tolled in honor of Manchester's own Ricky Hatton, who passed away 46 years old in recently.
It was the premier televised match card in the urban center since Hatton's passing. As the last bell disappeared, the silence transitioned to a roar of "there's only one Hatton".
The fighting during the bout equaled the sentiment. Arthur was sent to the canvas by a left hook flush on the jaw in the initial stage, rising shakily to his feet on the referee's count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was marked up quickly, heavy swelling forming under his face as the fighters exchanged punishing blows.
Rea, twenty-seven, landed power punches but the champion, thirty-four, fought fire with fire with powerful upward punches as the action rarely dropped over twelve rounds.
The victor narrowly won the decision with official scores of 114-114, narrow lead and clearer margin, but each competitor were relentless and full of heart, all the qualities their inspiration the legend embodied.
