British light-heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful individual punches in a fight where the techniques never quite clicked, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 for to the winner, with the third a even score.
"It wasn't my greatest display but what defines champions? They succeed," the victor commented in the ring as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Various analysts thought the challenger, 31, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-match professional journey.
The Champion Secures Triumph in Dull Affair
The fighter's advancement has slowed in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the future of UK boxing, he now finds himself trying to achieve expectations - possibly in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with neither fighter prepared to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super middleweight against John Ryder in 2022 after damaging his hand mid-fight, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to narrowly win the cleaner periods.
The audience's tolerance diminished amid frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor progresses, but with the likes of power puncher Benavidez, and Russian star Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Hatton Tribute Inspires Manchester Derby Battle
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to secure the continental championship.
The arena fell silent for a profoundly moving memorial prior to the bout as a bell ceremony tolled in memory of the city's native Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the final chime disappeared, the stillness transitioned to a chant of "there's only one Ricky Hatton".
The competition inside the ropes equaled the emotion. The eventual winner was knocked down by a left hook directly to the face in the early round, standing uncertainly to his feet on the count of eight.
But the red-colored attire of Arthur soon advanced again. From the other corner, Rea - in opposing blue - was injured quickly, significant bruising developing under his eye as the duo exchanged powerful shots.
Rea, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity hardly diminished over 12 rounds.
Arthur edged the result with scoring tallies of draw, 115-113 and wider gap, but the two athletes were persistent and courageous, everything their inspiration the legend embodied.
