UK light-heavyweight Buatsi secured a victory yet disappointing start to life under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's activity level likely gained him a split decision in a close ten-round contest at the boxing arena.
But the challenger had his moments, landing some telling clean blows in a contest where the approaches failed to mesh, with constant grabbing marring the action.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95 draw.
"It wasn't my best performance but how do champions respond? They win," Buatsi said in the ringside as he was greeted with boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Several boxing experts felt the challenger, thirty-one, merited the win but the contender experienced the second loss of his 28-fight career.
Buatsi Secures Win in Dull Affair
His career development has slowed in the past few years through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of UK boxing, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against John Ryder in 2022 after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective rounds.
The crowd's patience decreased amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for the challenger, giving Buatsi just only one stanza.
The victor moves on, but with the caliber of power puncher Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester contest to capture the European light-heavyweight belt.
The stadium fell silent for a deeply poignant memorial before the fight as a bell ceremony rang in remembrance of Manchester's own Ricky Hatton, who passed away at age 46 in September.
It was the initial significant broadcast fight night in the city since Hatton's death. As the last bell ended, the silence became a roar of "only one Hatton".
The fighting during the bout matched the feeling. Arthur was knocked down by a powerful punch directly to the face in the early round, getting up unsteadily to his feet on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was damaged quickly, heavy swelling developing under his eye as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, thirty-four, fought fire with fire with stinging uppercuts as the action rarely dropped over twelve rounds.
The victor barely secured the verdict with official scores of 114-114, close margin and 115-112, but each competitor were relentless and full of heart, all the qualities their hero the legend stood for.
