British light heavyweight Buatsi made a winning yet underwhelming start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a split decision in a competitive 10-round fight at the boxing arena.
But Parker had his moments, landing some telling single shots in a contest where the techniques never quite clicked, with excessive holding marring the contest.
Two judges judged it 96-94 to the winner, with the third a even score.
"It wasn't my best performance but how do champions respond? They prevail," the victor stated in the arena as he was met by disapproval from the crowd.
The Londoner bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to earn a 20th career victory.
Several boxing experts thought the challenger, thirty-one, deserved the win but the Derbyshire fighter suffered the second loss of his 28-bout career.
The Champion Earns Win in Uneventful Affair
The fighter's advancement has slowed in recent years through contract problems and long spells of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under fresh management.
He opened tentatively, with neither fighter willing to take risks in a tentative opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the middle rounds, the underdog had kept it close, causing problems for the favorite, who utilized a protective stance to narrowly win the more effective stanzas.
The spectators' acceptance diminished during frequent holding, with the challenger going down various instances during clinches.
Parker ended stronger, but many rounds were challenging to assess. Although, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi progresses, but with the presence of heavy hitter Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Tribute Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific all-Manchester contest to capture the European light heavyweight championship.
The venue grew hushed for a profoundly moving homage before the fight as a bell ceremony rang in memory of local hero Hatton, who succumbed aged 46 in last month.
It was the premier televised fight night in the location since the passing of Hatton. As the concluding sound ended, the stillness gave way to a shout of "there's just one Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a significant blow squarely on the chin in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. From the other corner, the opponent - in opposing blue - was damaged quickly, significant bruising developing under his orbital area as the pair traded punishing blows.
Rea, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the pace rarely dropped over the full distance.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and 115-112, but both fighters were relentless and full of heart, all the qualities their inspiration Hatton embodied.
