British light-heavyweight Buatsi made a winning yet disappointing start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a tight ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, landing some telling individual punches in a fight where the techniques never quite clicked, with excessive holding marring the contest.
The officials judged it 96-94 for to Buatsi, with the final judge a 95-95.
"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 crowd.
The British fighter returned to winning ways after his initial pro loss - an thrilling contest with his previous opponent in February - to claim a 20th career victory.
Various analysts felt the challenger, thirty-one, merited the decision but the British boxer endured the second loss of his 28-match career.
The Champion Earns Win in Dull Encounter
His career development has slowed in recent years through contract problems and extended periods of inactivity.
Previously predicted as the rising talent of British boxing, he now is positioned trying to achieve expectations - possibly in what appears to be a ultimate opportunity - under recent support.
He started carefully, with both competitors prepared to take risks in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was making just his second fight at light heavyweight.
By the middle rounds, Parker had made it competitive, creating difficulties for Buatsi, who worked behind a protective stance to barely take the more effective rounds.
The audience's tolerance diminished because of regular grabbing, with Parker falling to the floor multiple occasions during close exchanges.
Parker ended stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Contest
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a excellent all-Manchester battle to win the European light-heavyweight belt.
The arena fell silent for a extremely emotional tribute before the fight as a bell ceremony tolled in memory of the city's native Hatton, who succumbed aged 46 in September.
It was the premier televised boxing event in the urban center since Hatton's death. As the final chime disappeared, the silence became a roar of "there's just one Ricky Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was floored by a powerful punch directly to the jaw in the initial stage, rising shakily to his footing on the eight count.
But the Manchester United-red equipment of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was injured quickly, significant bruising appearing under his eye as the duo exchanged powerful shots.
Rea, 27, delivered effective strikes but Arthur, 34, responded in kind with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor narrowly won the result with official scores of draw, narrow lead and wider gap, but both fighters were determined and brave, all the qualities their inspiration Hatton stood for.
