British light heavyweight Buatsi earned a win yet underwhelming debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a majority decision in a close 10-round fight at the Manchester venue.
But the challenger landed significant shots, landing some telling single shots in a fight where the approaches didn't complement each other, with constant grabbing disrupting the contest.
The scoring judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They prevail," the victor stated in the arena as he was greeted with negative reactions from the spectators.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in last winter - to earn a twentieth professional win.
Various analysts believed the challenger, 31, should have been awarded the decision but the contender experienced the second setback of his 28-match professional journey.
The Champion Claims Victory in Slow Match
His career development has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors prepared to take risks in a calculated opening few rounds.
Parker, whose single earlier setback came at super middleweight against his former opponent in 2022 after damaging his hand mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who worked behind a protective stance to edge the more effective rounds.
The audience's tolerance wore thin because of regular grabbing, with the challenger slipping to the canvas various instances during grappling.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the likes of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Fuels Local Rivalry Battle
In the supporting bout, Lyndon Arthur defeated Bradley Rea by close verdict in a excellent all-Manchester battle to capture the continental belt.
The venue became quiet for a profoundly moving homage before the fight as a bell ceremony sounded in honor of local hero Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast boxing event in the city since the passing of Hatton. As the final chime disappeared, the silence became a roar of "only one Hatton".
The competition inside the ropes matched the emotion. The eventual winner was sent to the canvas by a significant blow flush on the jaw in the early round, getting up unsteadily to his feet on the count of eight.
But the red-colored equipment of Arthur soon pressed ahead again. Across the ring, Rea - in opposing blue - was injured quickly, significant bruising developing under his eye as the duo landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over the full distance.
The victor edged the decision with judges scorecards of 114-114, close margin and wider gap, but both fighters were relentless and brave, all the qualities their hero the legend stood for.
