British light-heavyweight Buatsi made a winning yet unimpressive start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective clean blows in a fight where the techniques failed to mesh, with excessive holding marring the contest.
Two judges judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my best display but what defines champions? They prevail," Buatsi commented in the ring as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a twentieth professional win.
Some ringside observers believed Parker, 31, should have been awarded the victory but the contender endured the second loss of his 28-fight career.
Buatsi Earns Victory in Uneventful Match
Buatsi's progress has slowed in recent times through contract problems and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now finds himself trying to realize his potential - maybe in what appears to be a last chance - under fresh management.
He started carefully, with both competitors willing to take risks in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at the higher weight class.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a protective stance to barely take the cleaner stanzas.
The audience's tolerance decreased because of frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Tribute Inspires Manchester Derby Fight
In the co-main event, Lyndon Arthur defeated Bradley Rea by close verdict in a outstanding all-Manchester contest to win the European light-heavyweight championship.
The arena grew hushed for a profoundly moving homage before the fight as 10 bells tolled in honor of Manchester's own Ricky Hatton, who passed away at age 46 in recently.
It was the initial significant broadcast match card in the location since Hatton's death. As the final chime faded, the stillness became a chant of "there's just one Hatton".
The fighting within the ring matched the sentiment. The eventual winner was floored by a powerful punch directly to the chin in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, the opponent - in competitive blue - was injured soon, heavy swelling appearing under his face as the pair exchanged punishing blows.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, responded in kind with powerful upward punches as the intensity hardly diminished over the full distance.
Arthur barely secured the verdict with official scores of 114-114, close margin and clearer margin, but the two athletes were persistent and brave, everything their idol the legend embodied.
