UK light heavyweight Buatsi made a winning yet disappointing beginning to life under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a split decision in a tight ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful clean blows in a fight where the approaches failed to mesh, with excessive holding disrupting the contest.
The officials had it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi stated in the ringside as he was greeted with disapproval from the spectators.
The British fighter got back to winning after his initial pro loss - an memorable fight with his previous opponent in February - to earn a 20th career victory.
Various analysts believed the challenger, 31, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-fight career.
Buatsi Secures Win in Dull Encounter
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Previously predicted as the rising talent of British boxing, he now finds himself trying to realize his potential - maybe in what seems like a last chance - under new promotional backing.
He started carefully, with both competitors ready to take risks in a tentative initial rounds.
The challenger, whose single earlier setback came at the lower weight class against his former opponent in 2022 after damaging his hand mid-fight, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who operated with a solid defense to edge the more effective rounds.
The crowd's patience decreased during constant clinching, with the challenger going down various instances during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
Buatsi progresses, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Motivates Manchester Derby Contest
In the supporting bout, Lyndon Arthur defeated Rea by divided decision in a outstanding all-Manchester fight to secure the continental belt.
The arena grew hushed for a extremely emotional homage ahead of the contest as ten bells rang in remembrance of Manchester's own Hatton, who passed away at age 46 in last month.
It was the premier televised fight night in the city since Hatton's death. As the last bell faded, the silence gave way to a shout of "there's only one Ricky Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a significant blow directly to the chin in the second round, rising shakily to his footing on the referee's count.
But the traditional red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up quickly, significant bruising developing under his face as the duo landed punishing blows.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over 12 rounds.
Arthur barely secured the result with official scores of 114-114, narrow lead and clearer margin, but each competitor were determined and brave, exactly what their inspiration Hatton represented.
