British light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a tight ten-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a fight where the techniques never quite clicked, with excessive holding disrupting the action.
The officials scored it 96-94 in favor to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what defines champions? They win," the victor said in the arena as he was met by boos from the crowd.
The Londoner got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to secure a twentieth professional win.
Various analysts thought the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter suffered the second setback of his 28-fight career.
The Champion Claims Triumph in Slow Encounter
Buatsi's progress has plateaued in the past few years through management issues and long spells of lack of fights.
Once tipped as the future of the British fight scene, he now stands trying to realize his potential - perhaps in what seems like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers prepared to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second bout at light heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for Buatsi, who utilized a solid defense to narrowly win the more effective periods.
The spectators' acceptance decreased amid constant clinching, with the challenger slipping to the canvas several times during grappling.
The underdog closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
Buatsi advances, but with the presence of heavy hitter David Benavidez, and Russian star Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Bradley Rea by divided decision in a terrific all-Manchester contest to capture the European light heavyweight belt.
The stadium became quiet for a deeply poignant tribute ahead of the contest as ten bells rang in honor of the city's native Hatton, who died at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound disappeared, the silence gave way to a roar of "only one Hatton".
The competition inside the ropes reflected the feeling. Arthur was sent to the canvas by a powerful punch squarely on the jaw in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured soon, heavy swelling developing under his orbital area as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over 12 rounds.
Arthur edged the result with scoring tallies of 114-114, 115-113 and wider gap, but each competitor were persistent and full of heart, everything their inspiration Hatton stood for.
