UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under fresh promotional backing with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a close ten-round bout at the Manchester venue.
But the challenger found success, delivering effective individual punches in a fight where the approaches didn't complement each other, with constant grabbing marring the fight.
The scoring judges scored it 96-94 for to the winner, with the third a 95-95.
"It wasn't my best display but how do champions respond? They prevail," Buatsi commented in the arena as he was faced negative reactions from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Various analysts felt the challenger, thirty-one, deserved the win but the contender experienced the second loss of his 28-fight career.
Buatsi Earns Triumph in Uneventful Match
The fighter's advancement has slowed in recent times through management issues and long spells of lack of fights.
Once tipped as the future of British boxing, he now is positioned trying to fulfil that early promise - perhaps in what seems like a final roll of the dice - under recent support.
He opened tentatively, with neither fighter prepared to be aggressive in a calculated initial rounds.
The challenger, whose only previous defeat came at super-middleweight against his former opponent in 2022 after suffering a hand injury in the contest, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, causing problems for Buatsi, who utilized a tight guard to barely take the cleaner periods.
The spectators' acceptance diminished amid frequent holding, with Parker going down various instances during grappling.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for Parker, scoring for Buatsi just a single round.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Manchester Derby Contest
In the co-main event, Lyndon Arthur beat Rea by divided decision in a terrific Manchester vs Manchester fight to win the continental belt.
The venue fell silent for a profoundly moving homage prior to the bout as 10 bells rang in remembrance of Manchester's own Hatton, who succumbed at age 46 in last month.
It was the initial significant broadcast match card in the urban center since the passing of Hatton. As the final chime faded, the silence transitioned to a roar of "only one Ricky Hatton".
The action within the ring equaled the feeling. The eventual winner was floored by a powerful punch flush on the jaw in the second round, rising shakily to his footing on the eight count.
But the Manchester United-red attire of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was damaged soon, significant bruising appearing under his eye as the duo exchanged hard punches.
Rea, 27, delivered effective strikes but the champion, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over 12 rounds.
The victor edged the result with judges scorecards of 114-114, close margin and 115-112, but the two athletes were persistent and full of heart, everything their inspiration Hatton stood for.
