British light heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a tight 10-round fight at the Co-op Live Arena.
But Parker had his moments, delivering effective clean blows in a match where the techniques never quite clicked, with frequent clinching disrupting the action.
The officials scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but what defines champions? They win," the victor stated in the ring as he was met by boos from the spectators.
The British fighter got back to winning after his maiden career setback - an epic battle with Callum Smith in February - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the decision but the contender endured the second defeat of his 28-fight career.
The Champion Claims Win in Uneventful Affair
Buatsi's progress has stalled in the past few years through management issues and considerable time of inactivity.
Previously predicted as the rising talent of UK boxing, he now stands trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under recent support.
He opened tentatively, with both competitors prepared to be aggressive in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second bout at light-heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who utilized a tight guard to narrowly win the more effective stanzas.
The crowd's patience diminished amid frequent holding, with the challenger going down several times during grappling.
The underdog finished the stronger, but several stanzas were hard to judge. Although, ex-champion the boxing analyst - commentating on the broadcast network - had it strongly leaning for Parker, giving Buatsi just a single round.
The victor advances, but with the caliber of hard hitter Benavidez, and Russian star Bivol ruling the division, the road to world glory - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires Manchester Derby Contest
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a outstanding Manchester vs Manchester contest to win the continental championship.
The stadium fell silent for a deeply poignant homage ahead of the contest as ten bells tolled in honor of the city's native Ricky Hatton, who passed away at age 46 in September.
It was the first major televised match card in the location since the passing of Hatton. As the final chime faded, the quiet transitioned to a chant of "only one Hatton".
The competition during the bout matched the feeling. The eventual winner was floored by a powerful punch directly to the face in the initial stage, rising shakily to his footing on the eight count.
But the traditional red shorts of Arthur soon pressed ahead again. From the other corner, the opponent - in opposing blue - was marked up quickly, significant bruising developing under his orbital area as the pair traded punishing blows.
Rea, twenty-seven, delivered effective strikes but the champion, 34, responded in kind with powerful upward punches as the action hardly diminished over 12 rounds.
Arthur narrowly won the verdict with judges scorecards of draw, 115-113 and wider gap, but each competitor were relentless and brave, exactly what their idol the legend represented.
