UK light-heavyweight Buatsi made a winning yet disappointing start to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a close 10-round bout at the Co-op Live Arena.
But Parker had his moments, connecting with powerful clean blows in a fight where the techniques didn't complement each other, with constant grabbing disrupting the fight.
The officials scored it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They win," Buatsi said in the ringside as he was faced disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an epic battle with Callum Smith in last winter - to earn a 20th pro success.
Several boxing experts felt the challenger, 31, merited the win but the Derbyshire fighter experienced the second loss of his 28-match career.
The Champion Secures Victory in Uneventful Affair
Buatsi's progress has stalled in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to take risks in a calculated initial rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who operated with a protective stance to barely take the cleaner rounds.
The audience's tolerance wore thin amid regular grabbing, with Parker slipping to the canvas various instances during grappling.
Parker closed well, but multiple periods were difficult to score. Nevertheless, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi advances, but with the presence of power puncher Benavidez, and Russian champion Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Memorial Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry fight to secure the European light heavyweight championship.
The arena grew hushed for a profoundly moving homage ahead of the contest as 10 bells tolled in memory of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised boxing event in the city since Hatton's death. As the last bell faded, the quiet gave way to a shout of "there's only one Ricky Hatton".
The action within the ring equaled the feeling. The eventual winner was knocked down by a left hook squarely on the chin in the initial stage, getting up unsteadily to his position on the referee's count.
But the traditional red equipment of the fighter soon came forward again. Across the ring, the opponent - in rival City blue - was injured early, noticeable damage forming under his orbital area as the fighters traded powerful shots.
Rea, 27, landed power punches but Arthur, thirty-four, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with judges scorecards of even score, 115-113 and 115-112, but each competitor were persistent and full of heart, everything their inspiration the legend represented.
