UK light-heavyweight Buatsi secured a victory yet disappointing start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a split decision in a competitive ten-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective individual punches in a fight where the styles didn't complement each other, with constant grabbing marring the contest.
The officials judged it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my greatest showing but what defines champions? They win," the victor stated in the ringside as he was faced negative reactions from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Claims Victory in Dull Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and extended periods of lack of fights.
Previously predicted as the rising talent of the British fight scene, he now stands trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He started carefully, with both competitors prepared to overcommit in a calculated initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for the favorite, who utilized a solid defense to narrowly win the cleaner stanzas.
The crowd's patience diminished during frequent holding, with Parker falling to the floor various instances during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still appears challenging.
Hatton Tribute Motivates All-Manchester Contest
In the co-main event, Lyndon Arthur beat Rea by close verdict in a terrific Manchester vs Manchester battle to win the European light-heavyweight title.
The arena fell silent for a extremely emotional homage prior to the bout as ten bells rang in honor of Manchester's own Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the city since the passing of Hatton. As the final chime ended, the quiet became a shout of "there's only one Hatton".
The competition within the ring reflected the sentiment. The eventual winner was floored by a powerful punch flush on the face in the early round, getting up unsteadily to his feet on the count of eight.
But the Manchester United-red attire of the fighter soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was marked up soon, heavy swelling forming under his eye as the fighters landed powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with stinging uppercuts as the pace rarely dropped over 12 rounds.
Arthur edged the verdict with judges scorecards of 114-114, close margin and 115-112, but the two athletes were determined and brave, everything their idol the legend stood for.
