British light heavyweight Buatsi made a winning yet disappointing beginning to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's activity level appeared to earn him a split decision in a competitive ten-round contest at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the techniques failed to mesh, with excessive holding disrupting the contest.
Two judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best performance but what do champions do? They win," the victor said in the arena as he was greeted with negative reactions from the crowd.
The Londoner got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, deserved the victory but the Derbyshire fighter endured the second defeat of his 28-bout professional journey.
The Champion Secures Win in Slow Encounter
His career development has plateaued in recent years through promotional disputes and extended periods of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a last chance - under new promotional backing.
He started carefully, with both competitors ready to overcommit in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against John Ryder in 2022 after damaging his hand in the contest, was competing in only his second fight at the higher weight class.
By the halfway stage, the underdog had made it competitive, making life awkward for Buatsi, who operated with a solid defense to barely take the more effective rounds.
The audience's tolerance decreased amid frequent holding, with Parker slipping to the canvas various instances during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just only one stanza.
The victor progresses, but with the presence of hard hitter David Benavidez, and Russian champion Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Hatton Memorial Inspires Local Rivalry Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester fight to capture the continental title.
The stadium fell silent for a deeply poignant homage before the fight as ten bells sounded in honor of Manchester's own Hatton, who died at age 46 in recently.
It was the first major televised fight night in the urban center since Hatton's death. As the last bell disappeared, the silence gave way to a chant of "there's only one Hatton".
The competition within the ring equaled the feeling. Arthur was knocked down by a powerful punch squarely on the face in the second round, getting up unsteadily to his footing on the eight count.
But the red-colored shorts of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was injured soon, heavy swelling forming under his orbital area as the pair exchanged powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, matched the intensity with stinging uppercuts as the action rarely dropped over the full distance.
The victor edged the verdict with official scores of even score, narrow lead and wider gap, but the two athletes were determined and brave, exactly what their idol Hatton stood for.
