British light heavyweight Buatsi earned a win yet disappointing start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a match where the techniques never quite clicked, with excessive holding disrupting the contest.
The officials had it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They prevail," Buatsi stated in the ring as he was faced disapproval from the spectators.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Some ringside observers thought Parker, 31, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-fight professional journey.
Buatsi Earns Win in Slow Encounter
The fighter's advancement has slowed in the past few years through contract problems and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to achieve expectations - perhaps in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors ready to overcommit in a cagey initial rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was making just his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, creating difficulties for Buatsi, who operated with a solid defense to edge the cleaner rounds.
The spectators' acceptance decreased because of regular grabbing, with the challenger falling to the floor several times during grappling.
The underdog finished the stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi advances, but with the caliber of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a outstanding all-Manchester fight to capture the European light-heavyweight title.
The arena grew hushed for a extremely emotional homage before the fight as ten bells sounded in memory of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast boxing event in the urban center since the passing of Hatton. As the final chime faded, the stillness became a roar of "there's just one Hatton".
The competition during the bout reflected the emotion. Arthur was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his position on the eight count.
But the red-colored shorts of Arthur soon advanced again. Across the ring, the opponent - in opposing blue - was damaged early, heavy swelling appearing under his orbital area as the duo traded hard punches.
The challenger, 27, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur barely secured the result with judges scorecards of 114-114, close margin and clearer margin, but the two athletes were persistent and full of heart, everything their idol the legend embodied.
