UK light heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a split decision in a close 10-round bout at the Manchester venue.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with excessive holding spoiling the contest.
The officials scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my best display but what defines champions? They succeed," Buatsi said in the arena as he was faced negative reactions from the audience.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers felt Parker, 31, merited the win but the Derbyshire fighter endured the second loss of his 28-fight career.
The Champion Claims Triumph in Slow Match
The fighter's advancement has slowed in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - perhaps in what feels like a final roll of the dice - under recent support.
He opened tentatively, with both competitors willing to be aggressive in a calculated opening few rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was making just his second appearance at the higher weight class.
By the middle rounds, Parker had kept it close, causing problems for the favorite, who utilized a tight guard to edge the better periods.
The spectators' acceptance decreased amid frequent holding, with Parker slipping to the canvas several times during grappling.
Parker closed well, but several stanzas were hard to judge. However, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just only one stanza.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian star Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates Manchester Derby Contest
In the secondary fight, Lyndon Arthur beat Rea by split-decision in a outstanding Manchester vs Manchester battle to secure the European light heavyweight belt.
The arena fell silent for a profoundly moving tribute prior to the bout as 10 bells sounded in memory of Manchester's own Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime ended, the silence transitioned to a roar of "there's just one Hatton".
The action within the ring reflected the feeling. The eventual winner was sent to the canvas by a left hook squarely on the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the red-colored equipment of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged early, noticeable damage developing under his orbital area as the duo traded powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the action hardly diminished over twelve rounds.
Arthur barely secured the verdict with official scores of 114-114, narrow lead and clearer margin, but both fighters were determined and full of heart, all the qualities their inspiration the legend stood for.
