British light-heavyweight Buatsi made a winning yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a tight 10-round bout at the Manchester venue.
But the challenger found success, delivering effective clean blows in a contest where the styles didn't complement each other, with excessive holding marring the fight.
The scoring judges judged it 96-94 for to the winner, with the final judge a even score.
"It wasn't my best display but how do champions respond? They prevail," Buatsi commented in the arena as he was met by boos from the crowd.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in February - to secure a 20th career victory.
Some ringside observers believed the challenger, 31, merited the win but the British boxer experienced the second setback of his 28-match professional journey.
The Champion Secures Win in Dull Affair
His career development has slowed in the past few years through contract problems and considerable time of inactivity.
Once tipped as the future of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with both competitors ready to take risks in a cagey early stages.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second fight at the higher weight class.
By the middle rounds, Parker had made it competitive, causing problems for the favorite, who operated with a protective stance to narrowly win the better periods.
The crowd's patience diminished during frequent holding, with the challenger falling to the floor several times during grappling.
Parker finished the stronger, but multiple periods were challenging to assess. Nevertheless, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, awarding Buatsi just only one stanza.
The victor progresses, but with the likes of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a outstanding all-Manchester contest to win the European light-heavyweight championship.
The stadium became quiet for a deeply poignant tribute prior to the bout as a bell ceremony sounded in memory of the city's native Ricky Hatton, who passed away at age 46 in recently.
It was the first major televised match card in the city since Hatton's passing. As the final chime disappeared, the silence transitioned to a chant of "only one Ricky Hatton".
The action within the ring equaled the sentiment. The eventual winner was knocked down by a significant blow directly to the jaw in the second round, getting up unsteadily to his footing on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was injured early, noticeable damage forming under his orbital area as the pair traded punishing blows.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity rarely dropped over the full distance.
The victor edged the decision with official scores of draw, narrow lead and clearer margin, but each competitor were relentless and brave, everything their inspiration the legend represented.
