UK light-heavyweight Buatsi secured a victory yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a split decision in a competitive ten-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a contest where the styles didn't complement each other, with excessive holding marring the action.
The scoring judges had it 96-94 in favor to the winner, with the third a even score.
"It wasn't my best display but what defines champions? They win," the victor said in the ringside as he was faced disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to claim a 20th career victory.
Several boxing experts thought the challenger, 31, should have been awarded the decision but the British boxer endured the second defeat of his 28-match professional journey.
The Champion Earns Win in Uneventful Affair
Buatsi's progress has plateaued in recent times through contract problems and extended periods of inactivity.
Earlier forecast as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what seems like a last chance - under recent support.
He opened tentatively, with neither fighter ready to take risks in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury in the contest, was competing in only his second appearance at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who operated with a protective stance to narrowly win the cleaner rounds.
The spectators' acceptance diminished amid frequent holding, with the challenger slipping to the canvas various instances during grappling.
Parker ended stronger, but many rounds were hard to judge. However, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Hatton Tribute Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific local rivalry contest to capture the European light-heavyweight championship.
The arena became quiet for a extremely emotional memorial prior to the bout as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell ended, the quiet became a chant of "there's just one Hatton".
The competition inside the ropes reflected the sentiment. The eventual winner was floored by a powerful punch directly to the face in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon advanced again. Across the ring, the opponent - in opposing blue - was injured early, significant bruising developing under his orbital area as the duo exchanged punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor edged the decision with official scores of even score, close margin and clearer margin, but the two athletes were relentless and courageous, everything their hero the legend represented.
