UK light heavyweight Buatsi earned a win yet underwhelming debut to life under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate seemingly secured him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a fight where the styles failed to mesh, with excessive holding spoiling the action.
Two judges had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my greatest display but how do champions respond? They succeed," the victor said in the ringside as he was faced disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th career victory.
Some ringside observers felt Parker, thirty-one, should have been awarded the victory but the British boxer endured the second defeat of his 28-bout professional journey.
Buatsi Claims Victory in Uneventful Match
The fighter's advancement has stalled in recent times through promotional disputes and extended periods of lack of fights.
Previously predicted as the future of the British fight scene, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a cagey initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, making life awkward for the favorite, who operated with a solid defense to edge the better rounds.
The crowd's patience wore thin during constant clinching, with Parker falling to the floor various instances during grappling.
Parker ended stronger, but multiple periods were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Hatton Memorial Motivates Local Rivalry Battle
In the supporting bout, Lyndon Arthur beat Rea by split-decision in a excellent all-Manchester fight to win the European light heavyweight title.
The arena fell silent for a profoundly moving homage prior to the bout as a bell ceremony rang in memory of the city's native Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the location since the passing of Hatton. As the concluding sound disappeared, the stillness became a chant of "only one Ricky Hatton".
The fighting during the bout equaled the sentiment. The eventual winner was sent to the canvas by a significant blow directly to the chin in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his face as the duo traded punishing blows.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with effective uppercuts as the intensity seldom decreased over twelve rounds.
Arthur barely secured the decision with official scores of 114-114, narrow lead and clearer margin, but the two athletes were relentless and brave, everything their inspiration the legend represented.
