British light heavyweight Buatsi secured a victory yet disappointing beginning to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a tight 10-round fight at the Manchester venue.
But the challenger found success, connecting with powerful clean blows in a contest where the techniques failed to mesh, with constant grabbing spoiling the action.
Two judges scored it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best performance but what do champions do? They prevail," Buatsi stated in the ringside as he was met by disapproval from the audience.
The British fighter bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a 20th pro success.
Some ringside observers believed the challenger, 31, deserved the decision but the Derbyshire fighter experienced the second loss of his 28-bout career.
The Champion Earns Victory in Uneventful Encounter
Buatsi's progress has stalled in the past few years through contract problems and extended periods of lack of fights.
Earlier forecast as the next star of UK boxing, he now finds himself trying to realize his potential - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter prepared to take risks in a cagey early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury in the contest, was making just his second bout at light heavyweight.
By the fight's midpoint, the underdog had kept it close, creating difficulties for the favorite, who operated with a protective stance to narrowly win the more effective periods.
The crowd's patience diminished because of frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, previous titleholder the boxing analyst - working with Dazn - had it particularly wide for Parker, scoring for Buatsi just a single round.
Buatsi advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Homage Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur beat Rea by split-decision in a terrific Manchester vs Manchester contest to win the continental belt.
The arena fell silent for a deeply poignant memorial before the fight as 10 bells sounded in remembrance of local hero Hatton, who succumbed aged 46 in last month.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell ended, the stillness gave way to a shout of "there's just one Hatton".
The competition within the ring reflected the feeling. Arthur was sent to the canvas by a left hook squarely on the chin in the early round, standing uncertainly to his footing on the eight count.
But the traditional red shorts of the fighter soon advanced again. From the other corner, Rea - in competitive blue - was damaged quickly, noticeable damage appearing under his orbital area as the fighters landed powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with powerful upward punches as the pace rarely dropped over twelve rounds.
Arthur edged the decision with judges scorecards of even score, 115-113 and wider gap, but both fighters were relentless and full of heart, everything their idol the legend represented.
