British light heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a close ten-round bout at the boxing arena.
But the challenger found success, connecting with powerful single shots in a fight where the styles never quite clicked, with frequent clinching spoiling the contest.
Two judges had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my greatest performance but what defines champions? They prevail," the victor stated in the ringside as he was greeted with boos from the crowd.
The British fighter returned to winning ways after his maiden career setback - an epic battle with his previous opponent in February - to secure a 20th career victory.
Some ringside observers thought Parker, 31, should have been awarded the decision but the British boxer endured the second loss of his 28-match professional journey.
Buatsi Secures Win in Uneventful Affair
Buatsi's progress has plateaued in recent years through contract problems and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at the higher weight class.
By the fight's midpoint, Parker had kept it close, causing problems for the favorite, who worked behind a protective stance to narrowly win the more effective stanzas.
The audience's tolerance diminished amid regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
The underdog ended stronger, but several stanzas were difficult to score. Although, former world champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor moves on, but with the likes of heavy hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels Local Rivalry Contest
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding local rivalry battle to win the European light heavyweight belt.
The arena fell silent for a profoundly moving homage prior to the bout as 10 bells tolled in memory of the city's native Hatton, who passed away 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound disappeared, the quiet gave way to a roar of "only one Ricky Hatton".
The action inside the ropes reflected the feeling. Arthur was sent to the canvas by a significant blow directly to the face in the second round, standing uncertainly to his position on the referee's count.
But the traditional red shorts of Arthur soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, heavy swelling appearing under his orbital area as the duo exchanged hard punches.
The challenger, 27, connected with strong blows but the champion, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over 12 rounds.
Arthur edged the result with scoring tallies of 114-114, narrow lead and clearer margin, but both fighters were determined and courageous, everything their inspiration the legend stood for.
