British light heavyweight Buatsi secured a victory yet unimpressive start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a close ten-round bout at the Manchester venue.
But Parker had his moments, connecting with powerful individual punches in a contest where the techniques failed to mesh, with constant grabbing spoiling the contest.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They prevail," the victor said in the ring as he was greeted with negative reactions from the spectators.
The British fighter got back to winning after his initial pro loss - an thrilling contest with his previous opponent in last winter - to claim a twentieth professional win.
Some ringside observers felt Parker, thirty-one, deserved the win but the British boxer endured the second setback of his 28-bout career.
Buatsi Claims Victory in Dull Match
His career development has plateaued in recent times through management issues and extended periods of lack of fights.
Once tipped as the rising talent of British boxing, he now stands trying to fulfil that early promise - possibly in what feels like a last chance - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after suffering a hand injury mid-fight, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, causing problems for Buatsi, who worked behind a solid defense to edge the more effective rounds.
The spectators' acceptance decreased amid regular grabbing, with Parker going down multiple occasions during grappling.
The underdog ended stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for the challenger, awarding Buatsi just a single round.
The victor advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Manchester Derby Fight
In the co-main event, Arthur beat Rea by divided decision in a outstanding local rivalry fight to capture the continental belt.
The venue grew hushed for a deeply poignant tribute ahead of the contest as ten bells tolled in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the final chime disappeared, the quiet gave way to a shout of "there's just one Ricky Hatton".
The action inside the ropes matched the emotion. The eventual winner was floored by a powerful punch directly to the jaw in the second round, getting up unsteadily to his position on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was marked up quickly, noticeable damage forming under his eye as the duo landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over twelve rounds.
The victor edged the result with official scores of even score, narrow lead and wider gap, but each competitor were determined and courageous, everything their inspiration the legend stood for.
