British light heavyweight Buatsi earned a win yet underwhelming beginning to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger landed significant shots, landing some telling clean blows in a match where the approaches didn't complement each other, with excessive holding marring the fight.
The officials scored it 96-94 to Buatsi, with the remaining official a 95-95.
"It wasn't my greatest performance but what do champions do? They succeed," the victor commented in the ringside as he was met by boos from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to secure a 20th pro success.
Several boxing experts thought the challenger, 31, deserved the decision but the contender experienced the second setback of his 28-fight career.
Buatsi Claims Triumph in Slow Affair
The fighter's advancement has stalled in recent years through contract problems and long spells of lack of fights.
Previously predicted as the rising talent of British boxing, he now stands trying to realize his potential - maybe in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with the two boxers ready to be aggressive in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after breaking his hand mid-fight, was participating in his second bout at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to barely take the cleaner stanzas.
The crowd's patience decreased because of frequent holding, with the challenger falling to the floor various instances during grappling.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still appears challenging.
Ricky Hatton Tribute Fuels All-Manchester Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester contest to secure the European light-heavyweight belt.
The stadium fell silent for a deeply poignant homage ahead of the contest as 10 bells sounded in remembrance of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the initial significant broadcast boxing event in the urban center since Hatton's death. As the last bell faded, the quiet became a roar of "there's only one Ricky Hatton".
The competition inside the ropes equaled the feeling. Arthur was knocked down by a powerful punch directly to the jaw in the early round, rising shakily to his feet on the count of eight.
But the traditional red shorts of Arthur soon pressed ahead again. On the opposite side, the opponent - in competitive blue - was damaged quickly, noticeable damage developing under his eye as the duo traded powerful shots.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
Arthur edged the result with official scores of even score, 115-113 and 115-112, but both fighters were determined and brave, everything their hero Hatton stood for.
