UK light heavyweight Buatsi secured a victory yet unimpressive debut to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic medal winner's activity level seemingly secured him a split decision in a close 10-round bout at the boxing arena.
But the challenger had his moments, delivering effective clean blows in a match where the styles failed to mesh, with excessive holding marring the fight.
Two judges had it 96-94 to the winner, with the third a even score.
"It wasn't my best performance but what defines champions? They win," the victor said in the ring as he was met by boos from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Some ringside observers believed Parker, 31, deserved the decision but the contender suffered the second setback of his 28-fight career.
Buatsi Claims Victory in Uneventful Affair
Buatsi's progress has slowed in recent years through management issues and long spells of inactivity.
Once tipped as the next star of the British fight scene, he now is positioned trying to realize his potential - possibly in what seems like a last chance - under fresh management.
He started carefully, with both competitors ready to take risks in a calculated opening few rounds.
Parker, whose sole prior loss came at super middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for the favorite, who utilized a solid defense to edge the better rounds.
The spectators' acceptance decreased during frequent holding, with the challenger falling to the floor various instances during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the presence of heavy hitter Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to secure the continental belt.
The arena fell silent for a profoundly moving homage before the fight as 10 bells sounded in memory of the city's native Ricky Hatton, who died 46 years old in recently.
It was the first major televised boxing event in the location since Hatton's death. As the final chime faded, the quiet gave way to a roar of "only one Ricky Hatton".
The action during the bout equaled the emotion. The eventual winner was sent to the canvas by a left hook squarely on the jaw in the initial stage, getting up unsteadily to his feet on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, Rea - in rival City blue - was damaged quickly, heavy swelling appearing under his face as the pair exchanged hard punches.
Rea, twenty-seven, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over twelve rounds.
Arthur edged the verdict with judges scorecards of even score, 115-113 and wider gap, but the two athletes were determined and courageous, all the qualities their inspiration Hatton embodied.
