UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But the challenger found success, connecting with powerful single shots in a fight where the techniques didn't complement each other, with constant grabbing disrupting the action.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my greatest performance but what do champions do? They win," the victor said in the ring as he was faced negative reactions from the audience.
The Londoner bounced back after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts thought the challenger, 31, merited the decision but the contender suffered the second loss of his 28-bout professional journey.
Buatsi Secures Triumph in Slow Affair
The fighter's advancement has stalled in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of British boxing, he now finds himself trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under fresh management.
He began cautiously, with the two boxers ready to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had made it competitive, causing problems for the favorite, who operated with a tight guard to edge the better stanzas.
The crowd's patience decreased amid frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog closed well, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor advances, but with the caliber of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific local rivalry fight to secure the European light-heavyweight championship.
The stadium fell silent for a profoundly moving tribute prior to the bout as a bell ceremony tolled in memory of local hero Hatton, who passed away at age 46 in recently.
It was the first major televised match card in the city since the passing of Hatton. As the last bell ended, the quiet became a roar of "there's only one Hatton".
The competition during the bout equaled the feeling. Arthur was floored by a significant blow flush on the face in the early round, rising shakily to his footing on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. From the other corner, Rea - in competitive blue - was marked up quickly, heavy swelling forming under his orbital area as the fighters traded hard punches.
The challenger, twenty-seven, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace rarely dropped over 12 rounds.
Arthur edged the decision with official scores of 114-114, close margin and clearer margin, but the two athletes were relentless and brave, everything their hero Hatton represented.
