UK light heavyweight Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a competitive ten-round fight at the boxing arena.
But Parker had his moments, landing some telling clean blows in a contest where the styles didn't complement each other, with excessive holding marring the action.
The scoring judges scored it 96-94 in favor to Buatsi, with the final judge a 95-95 draw.
"It wasn't my greatest display but how do champions respond? They win," the victor commented in the ringside as he was greeted with negative reactions from the spectators.
The British fighter bounced back after his initial pro loss - an memorable fight with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers felt the challenger, 31, merited the win but the contender experienced the second loss of his 28-match career.
Buatsi Secures Win in Dull Match
Buatsi's progress has plateaued in the past few years through contract problems and extended periods of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to achieve expectations - perhaps in what seems like a final roll of the dice - under fresh management.
He started carefully, with neither fighter ready to take risks in a calculated early stages.
Parker, whose only previous defeat came at super-middleweight against John Ryder in two years ago after suffering a hand injury during the bout, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, creating difficulties for the favorite, who operated with a protective stance to edge the more effective periods.
The crowd's patience decreased during frequent holding, with Parker falling to the floor multiple occasions during clinches.
The underdog closed well, but multiple periods were difficult to score. However, former world champion the boxing analyst - working with Dazn - had it strongly leaning for Parker, giving Buatsi just a single round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Hatton Homage Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Rea by close verdict in a outstanding all-Manchester battle to secure the European light heavyweight championship.
The stadium fell silent for a profoundly moving homage before the fight as ten bells rang in remembrance of the city's native Hatton, who passed away 46 years old in last month.
It was the premier televised boxing event in the location since the passing of Hatton. As the final chime ended, the stillness transitioned to a roar of "there's only one Hatton".
The action within the ring reflected the emotion. Arthur was knocked down by a significant blow directly to the face in the early round, getting up unsteadily to his position on the eight count.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was damaged soon, significant bruising appearing under his eye as the fighters traded powerful shots.
Rea, 27, delivered effective strikes but Arthur, thirty-four, matched the intensity with powerful upward punches as the action rarely dropped over the full distance.
The victor barely secured the verdict with official scores of 114-114, 115-113 and clearer margin, but both fighters were persistent and brave, everything their inspiration Hatton represented.
