UK light heavyweight Buatsi secured a victory yet disappointing beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close 10-round contest at the Co-op Live Arena.
But Parker had his moments, landing some telling clean blows in a contest where the styles never quite clicked, with excessive holding disrupting the contest.
Two judges scored it 96-94 for to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest showing but what do champions do? They win," the victor said in the arena as he was faced boos from the crowd.
The British fighter got back to winning after his initial pro loss - an thrilling contest with Callum Smith in February - to secure a 20th career victory.
Some ringside observers thought the challenger, thirty-one, deserved the decision but the British boxer experienced the second loss of his 28-fight professional journey.
The Champion Claims Win in Uneventful Affair
His career development has plateaued in the past few years through management issues and long spells of inactivity.
Once tipped as the next star of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with the two boxers ready to be aggressive in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to edge the better periods.
The audience's tolerance diminished during frequent holding, with the challenger going down various instances during close exchanges.
Parker finished the stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still seems difficult.
Hatton Homage Motivates Local Rivalry Contest
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding Manchester vs Manchester contest to capture the continental belt.
The stadium grew hushed for a profoundly moving tribute before the fight as 10 bells tolled in honor of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's passing. As the concluding sound ended, the quiet became a chant of "there's only one Hatton".
The competition during the bout matched the feeling. Arthur was sent to the canvas by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the traditional red shorts of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was damaged early, significant bruising developing under his face as the pair traded powerful shots.
Rea, 27, delivered effective strikes but the champion, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur barely secured the result with official scores of draw, narrow lead and 115-112, but the two athletes were persistent and full of heart, everything their idol Hatton represented.
