UK light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round contest at the boxing arena.
But the challenger found success, landing some telling single shots in a match where the styles failed to mesh, with excessive holding spoiling the contest.
The scoring judges had it 96-94 in favor to the winner, with the remaining official a 95-95.
"It wasn't my best display but what do champions do? They win," the victor commented in the ringside as he was greeted with negative reactions from the spectators.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to secure a 20th pro success.
Some ringside observers felt Parker, thirty-one, deserved the decision but the British boxer experienced the second setback of his 28-match career.
The Champion Secures Triumph in Uneventful Match
His career development has slowed in the past few years through contract problems and long spells of lack of fights.
Previously predicted as the next star of British boxing, he now finds himself trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to be aggressive in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after breaking his hand in the contest, was participating in his second appearance at light heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to edge the more effective periods.
The audience's tolerance decreased during frequent holding, with Parker slipping to the canvas multiple occasions during clinches.
The underdog ended stronger, but several stanzas were challenging to assess. Although, ex-champion Carl Frampton - commentating on Dazn - had it particularly wide for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of power puncher Benavidez, and Russian star Bivol ruling the division, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Homage Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur defeated Rea by split-decision in a outstanding local rivalry battle to secure the European light heavyweight championship.
The venue became quiet for a profoundly moving tribute before the fight as 10 bells sounded in remembrance of local hero Ricky Hatton, who passed away 46 years old in September.
It was the first major televised match card in the city since Hatton's death. As the final chime faded, the quiet gave way to a roar of "only one Hatton".
The fighting during the bout equaled the emotion. The eventual winner was floored by a left hook flush on the chin in the second round, standing uncertainly to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. From the other corner, Rea - in opposing blue - was damaged early, significant bruising forming under his eye as the fighters traded powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, fought fire with fire with effective uppercuts as the action hardly diminished over twelve rounds.
Arthur barely secured the decision with official scores of 114-114, narrow lead and clearer margin, but both fighters were determined and full of heart, all the qualities their hero the legend represented.
