UK light-heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a tight ten-round bout at the Co-op Live Arena.
But Parker had his moments, landing some telling individual punches in a match where the techniques didn't complement each other, with constant grabbing disrupting the fight.
The officials had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but what defines champions? They succeed," Buatsi commented in the arena as he was faced negative reactions from the spectators.
The Londoner returned to winning ways after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th career victory.
Some ringside observers felt Parker, thirty-one, deserved the victory but the Derbyshire fighter suffered the second loss of his 28-bout career.
Buatsi Claims Victory in Dull Match
Buatsi's progress has slowed in recent times through management issues and considerable time of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now is positioned trying to realize his potential - perhaps in what seems like a last chance - under new promotional backing.
He opened tentatively, with the two boxers ready to take risks in a cagey early stages.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for the favorite, who operated with a tight guard to narrowly win the more effective rounds.
The spectators' acceptance decreased because of frequent holding, with the challenger slipping to the canvas several times during grappling.
The underdog finished the stronger, but several stanzas were hard to judge. However, previous titleholder the boxing analyst - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
Buatsi progresses, but with the caliber of hard hitter David Benavidez, and Russian star Bivol leading the category, the road to world glory - judging by this showing - still seems difficult.
Ricky Hatton Homage Fuels All-Manchester Battle
In the co-main event, Lyndon Arthur defeated Rea by split-decision in a outstanding all-Manchester contest to capture the continental title.
The arena fell silent for a extremely emotional memorial prior to the bout as ten bells sounded in memory of local hero Ricky Hatton, who died 46 years old in last month.
It was the first major televised boxing event in the city since Hatton's death. As the concluding sound faded, the silence became a roar of "there's just one Hatton".
The competition inside the ropes reflected the emotion. Arthur was knocked down by a left hook directly to the chin in the early round, standing uncertainly to his feet on the referee's count.
But the traditional red attire of the fighter soon came forward again. On the opposite side, Rea - in opposing blue - was damaged early, heavy swelling forming under his orbital area as the pair landed hard punches.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the result with judges scorecards of 114-114, narrow lead and wider gap, but each competitor were relentless and brave, everything their hero Hatton represented.
