British light heavyweight Buatsi secured a victory yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a majority decision in a close ten-round contest at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a contest where the styles never quite clicked, with constant grabbing disrupting the contest.
Two judges had it 96-94 to the winner, with the third a 95-95.
"It wasn't my finest performance but how do champions respond? They succeed," the victor stated in the ringside as he was met by disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a twentieth professional win.
Various analysts believed the challenger, thirty-one, merited the win but the Derbyshire fighter suffered the second defeat of his 28-bout career.
The Champion Earns Win in Slow Encounter
His career development 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 stands trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He began cautiously, with neither fighter ready to take risks in a tentative early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in 2022 after damaging his hand in the contest, was participating in his second appearance at light-heavyweight.
By the middle rounds, Parker had made it competitive, causing problems for Buatsi, who operated with a protective stance to narrowly win the cleaner periods.
The audience's tolerance wore thin during regular grabbing, with Parker falling to the floor several times during close exchanges.
The underdog closed well, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the likes of power puncher David Benavidez, and Russian champion Bivol dominating the weight class, the road to world glory - based on this performance - still appears challenging.
Hatton Homage Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a excellent all-Manchester battle to capture the European light-heavyweight title.
The stadium fell silent for a profoundly moving memorial before the fight as a bell ceremony sounded in memory of Manchester's own Hatton, who succumbed aged 46 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 roar of "only one Ricky Hatton".
The action during the bout reflected the sentiment. Arthur was knocked down by a left hook squarely on the face in the initial stage, getting up unsteadily to his footing on the count of eight.
But the red-colored attire of the fighter soon advanced again. Across the ring, the opponent - in rival City blue - was marked up soon, noticeable damage appearing under his eye as the fighters traded powerful shots.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action seldom decreased over twelve rounds.
Arthur edged the verdict with official scores of draw, close margin and wider gap, but the two athletes were determined and courageous, exactly what their inspiration the legend embodied.
