UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output seemingly secured him a judges' verdict in a tight 10-round contest at the boxing arena.
But the challenger had his moments, delivering effective individual punches in a fight where the styles never quite clicked, with constant grabbing disrupting the contest.
Two judges judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest showing but what defines champions? They prevail," the victor stated in the arena as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in last winter - to claim a 20th pro success.
Some ringside observers believed the challenger, 31, should have been awarded the win but the Derbyshire fighter endured the second setback of his 28-fight career.
Buatsi Earns Triumph in Uneventful Encounter
Buatsi's progress has plateaued in recent years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of UK boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with neither fighter willing to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light-heavyweight.
By the middle rounds, the underdog had made it competitive, making life awkward for Buatsi, who operated with a solid defense to edge the cleaner periods.
The audience's tolerance decreased during regular grabbing, with the challenger slipping to the canvas several times during clinches.
The underdog finished the stronger, but many rounds were difficult to score. However, former world champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor advances, but with the likes of heavy hitter Benavidez, and Russian champion Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Motivates Local Rivalry Contest
In the co-main event, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific local rivalry contest to capture the European light heavyweight title.
The arena fell silent for a extremely emotional tribute ahead of the contest as ten bells tolled in memory of the city's native Ricky Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the urban center since Hatton's passing. As the concluding sound ended, the quiet became a shout of "there's just one Hatton".
The action inside the ropes equaled the feeling. Arthur was floored by a powerful punch directly to the chin in the second round, standing uncertainly to his footing on the referee's count.
But the red-colored equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up soon, significant bruising forming under his eye as the duo landed punishing blows.
Rea, 27, connected with strong blows but Arthur, thirty-four, responded in kind with powerful upward punches as the pace hardly diminished over twelve rounds.
The victor edged the decision with official scores of draw, narrow lead and clearer margin, but both fighters were determined and full of heart, all the qualities their idol the legend stood for.
