UK light heavyweight Buatsi made a winning yet unimpressive debut to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a competitive 10-round bout at the boxing arena.
But the challenger had his moments, connecting with powerful single shots in a fight where the approaches didn't complement each other, with frequent clinching spoiling the contest.
The officials had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," Buatsi commented in the arena as he was greeted with boos from the audience.
The Londoner got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers believed Parker, 31, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-match career.
Buatsi Earns Triumph in Slow Match
The fighter's advancement has plateaued in recent years through contract problems and long spells of lack of fights.
Previously predicted as the future of UK boxing, he now finds himself trying to realize his potential - maybe in what seems like a last chance - under recent support.
He started carefully, with both competitors ready to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was making just his second appearance at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, causing problems for the favorite, who worked behind a solid defense to barely take the cleaner stanzas.
The crowd's patience wore thin because of frequent holding, with the challenger slipping to the canvas various instances during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. However, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Contest
In the co-main event, Arthur overcame Bradley Rea by split-decision in a excellent local rivalry battle to win the European light-heavyweight title.
The venue fell silent for a extremely emotional homage ahead of the contest as 10 bells sounded in remembrance of Manchester's own Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's death. As the concluding sound disappeared, the quiet transitioned to a shout of "only one Ricky Hatton".
The competition inside the ropes equaled the sentiment. The eventual winner was sent to the canvas by a left hook squarely on the chin in the initial stage, rising shakily to his footing on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was marked up quickly, heavy swelling appearing under his face as the pair exchanged punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with stinging uppercuts as the pace rarely dropped over the full distance.
The victor barely secured the result with official scores of 114-114, narrow lead and clearer margin, but both fighters were relentless and brave, all the qualities their idol Hatton embodied.
