British light-heavyweight Buatsi secured a victory yet unimpressive debut to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a close ten-round contest at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a fight where the techniques never quite clicked, with constant grabbing marring the contest.
The officials had it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest display but what do champions do? They prevail," the victor commented in the ring as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an thrilling contest with Callum Smith in February - to secure a twentieth professional win.
Various analysts felt the challenger, thirty-one, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Secures Win in Dull Encounter
The fighter's advancement has slowed in the past few years through management issues and long spells of inactivity.
Once tipped as the future of British boxing, he now stands trying to achieve expectations - maybe in what seems like a last chance - under new promotional backing.
He started carefully, with the two boxers willing to overcommit in a calculated opening few rounds.
Parker, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand mid-fight, was participating in his second bout at light heavyweight.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to edge the more effective periods.
The audience's tolerance decreased amid constant clinching, with the challenger going down several times during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. However, former world champion the boxing analyst - commentating on the broadcast network - had it significantly in favor for Parker, awarding Buatsi just one round.
Buatsi advances, but with the presence of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Homage Motivates All-Manchester Battle
In the supporting bout, Lyndon Arthur overcame Rea by close verdict in a terrific all-Manchester fight to win the European light heavyweight belt.
The venue fell silent for a deeply poignant tribute ahead of the contest as a bell ceremony rang in memory of the city's native Ricky Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime faded, the quiet became a chant of "there's only one Ricky Hatton".
The action inside the ropes equaled the sentiment. Arthur was knocked down by a significant blow directly to the face in the second round, getting up unsteadily to his footing on the count of eight.
But the red-colored shorts of Arthur soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged soon, heavy swelling developing under his eye as the pair exchanged hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the action rarely dropped over twelve rounds.
Arthur narrowly won the verdict with official scores of even score, close margin and 115-112, but both fighters were relentless and courageous, exactly what their hero Hatton stood for.
