UK light heavyweight Buatsi earned a win yet underwhelming debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's activity level likely gained him a judges' verdict in a tight 10-round fight at the Co-op Live Arena.
But the challenger had his moments, delivering effective clean blows in a match where the approaches didn't complement each other, with frequent clinching marring the contest.
The scoring judges judged it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but how do champions respond? They win," Buatsi said in the ringside as he was greeted with disapproval from the spectators.
The Londoner bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers thought the challenger, thirty-one, merited the win but the Derbyshire fighter suffered the second loss of his 28-match professional journey.
The Champion Claims Triumph in Dull Match
Buatsi's progress has slowed in recent times through promotional disputes and extended periods of lack of fights.
Earlier forecast as the rising talent of British boxing, he now stands trying to fulfil that early promise - maybe in what seems like a last chance - under fresh management.
He began cautiously, with neither fighter prepared to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against John Ryder in 2022 after suffering a hand injury mid-fight, was competing in only his second fight at the higher weight class.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who worked behind a solid defense to narrowly win the more effective stanzas.
The audience's tolerance diminished amid constant clinching, with the challenger going down multiple occasions during clinches.
The underdog ended stronger, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - providing analysis for Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - judging by this showing - still looks steep.
Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a excellent Manchester vs Manchester battle to capture the continental championship.
The venue fell silent for a deeply poignant memorial prior to the bout as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who died at age 46 in recently.
It was the first major televised match card in the location since the passing of Hatton. As the last bell disappeared, the silence transitioned to a roar of "there's just one Ricky Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was knocked down by a significant blow flush on the jaw in the initial stage, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of the fighter soon came forward again. On the opposite side, the opponent - in competitive blue - was injured soon, heavy swelling forming under his eye as the fighters landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the action seldom decreased over 12 rounds.
Arthur edged the result with official scores of 114-114, narrow lead and 115-112, but each competitor were relentless and full of heart, everything their idol the legend represented.
