British light-heavyweight Buatsi made a winning yet unimpressive beginning to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a split decision in a tight 10-round contest at the boxing arena.
But Parker found success, delivering effective clean blows in a contest where the techniques never quite clicked, with constant grabbing spoiling the contest.
The officials had it 96-94 to Buatsi, with the third a even score.
"It wasn't my best performance but what do champions do? They win," Buatsi stated in the arena as he was met by disapproval from the spectators.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th career victory.
Various analysts thought the challenger, thirty-one, merited the decision but the British boxer suffered the second defeat of his 28-match career.
Buatsi Secures Win in Dull Match
His career development has plateaued in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to achieve expectations - possibly in what seems like a last chance - under new promotional backing.
He opened tentatively, with both competitors prepared to overcommit in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for Buatsi, who operated with a protective stance to narrowly win the better rounds.
The crowd's patience decreased because of frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog ended stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the path to championship success - judging by this showing - still appears challenging.
Hatton Homage Motivates Manchester Derby Contest
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a outstanding Manchester vs Manchester contest to capture the European light-heavyweight championship.
The venue grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony sounded in honor of Manchester's own Ricky Hatton, who died aged 46 in last month.
It was the premier televised fight night in the location since Hatton's death. As the concluding sound faded, the silence became a roar of "there's just one Ricky Hatton".
The fighting inside the ropes matched the feeling. Arthur was sent to the canvas by a significant blow flush on the jaw in the early round, standing uncertainly to his feet on the referee's count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, the opponent - in competitive blue - was injured quickly, significant bruising developing under his eye as the duo exchanged hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, responded in kind with effective uppercuts as the pace hardly diminished over 12 rounds.
Arthur barely secured the result with scoring tallies of 114-114, 115-113 and clearer margin, but the two athletes were relentless and full of heart, all the qualities their idol the legend embodied.
