UK light heavyweight Buatsi earned a win yet disappointing beginning to his career under recent management with a decision win over Parker in the Manchester arena.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close 10-round bout at the boxing arena.
But the challenger found success, delivering effective single shots in a match where the approaches didn't complement each other, with excessive holding spoiling the contest.
The scoring judges had it 96-94 to Buatsi, with the final judge a even score.
"It wasn't my best performance but what do champions do? They win," the victor commented in the ringside as he was greeted with disapproval from the audience.
The Londoner bounced back after his initial pro loss - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts believed Parker, 31, deserved the win but the Derbyshire fighter experienced the second loss of his 28-match career.
The Champion Earns Victory in Uneventful Encounter
Buatsi's progress has stalled in the past few years through promotional disputes and considerable time of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors willing to be aggressive in a cagey opening few rounds.
The challenger, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second bout at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the better rounds.
The spectators' acceptance decreased amid constant clinching, with Parker slipping to the canvas several times during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, former world champion Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just one round.
The victor moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - based on this performance - still looks steep.
Hatton Memorial Inspires Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by split-decision in a outstanding Manchester vs Manchester battle to capture the European light heavyweight belt.
The venue became quiet for a deeply poignant homage ahead of the contest as ten bells rang in remembrance of the city's native Hatton, who succumbed at age 46 in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the last bell ended, the quiet became a chant of "there's just one Hatton".
The action within the ring reflected the emotion. The eventual winner was sent to the canvas by a powerful punch directly to the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, the opponent - in competitive blue - was injured quickly, heavy swelling forming under his eye as the pair exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, thirty-four, matched the intensity with stinging uppercuts as the pace hardly diminished over the full distance.
Arthur edged the verdict with judges scorecards of 114-114, close margin and wider gap, but the two athletes were persistent and full of heart, exactly what their inspiration Hatton embodied.
