UK light heavyweight Buatsi earned a win yet underwhelming beginning to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a competitive ten-round bout at the boxing arena.
But the challenger landed significant shots, delivering effective individual punches in a contest where the approaches failed to mesh, with frequent clinching spoiling the contest.
Two judges had it 96-94 to the winner, with the third a 95-95 draw.
"It wasn't my best showing but what defines champions? They succeed," the victor said in the ringside as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to secure a twentieth professional win.
Some ringside observers felt the challenger, 31, deserved the decision but the Derbyshire fighter suffered the second loss of his 28-match career.
The Champion Claims Victory in Slow Encounter
Buatsi's progress has plateaued in the past few years through promotional disputes 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 - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter willing to overcommit in a tentative early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had kept it close, causing problems for the favorite, who worked behind a tight guard to narrowly win the better rounds.
The audience's tolerance diminished amid regular grabbing, with the challenger falling to the floor multiple occasions during grappling.
Parker closed well, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just a single round.
The victor advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still seems difficult.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester contest to win the European light heavyweight belt.
The venue fell silent for a extremely emotional memorial prior to the bout as ten bells tolled in remembrance of Manchester's own Hatton, who died at age 46 in September.
It was the initial significant broadcast fight night in the location since the passing of Hatton. As the last bell disappeared, the quiet became a shout of "there's only one Hatton".
The action inside the ropes matched the emotion. Arthur was floored by a powerful punch flush on the chin in the initial stage, rising shakily to his position on the eight count.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, the opponent - in competitive blue - was injured quickly, noticeable damage developing under his face as the pair traded hard punches.
Rea, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over twelve rounds.
Arthur barely secured the verdict with scoring tallies of draw, 115-113 and clearer margin, but the two athletes were relentless and courageous, exactly what their hero Hatton stood for.
