UK light-heavyweight Buatsi secured a victory yet disappointing beginning to life under new promoters with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a split decision in a tight ten-round fight at the Manchester venue.
But Parker found success, connecting with powerful individual punches in a match where the approaches never quite clicked, with excessive holding marring the fight.
Two judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my greatest showing but what do champions do? They prevail," Buatsi commented in the ring as he was greeted with negative reactions from the audience.
The British fighter returned to winning ways after his initial pro loss - an epic battle with his previous opponent in last winter - to earn a 20th pro success.
Various analysts believed Parker, 31, should have been awarded the win but the Derbyshire fighter suffered the second loss of his 28-fight career.
Buatsi Secures Triumph in Slow Match
The fighter's advancement has slowed in recent times through promotional disputes and considerable time of inactivity.
Earlier forecast as the future of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He began cautiously, with both competitors willing to be aggressive in a tentative early stages.
Parker, whose single earlier setback came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second bout at the higher weight class.
By the middle rounds, the underdog had made it competitive, creating difficulties for the favorite, who operated with a solid defense to barely take the better rounds.
The crowd's patience decreased amid constant clinching, with Parker falling to the floor several times during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. Although, ex-champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - on this evidence - still appears challenging.
Hatton Memorial Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur defeated Rea by close verdict in a terrific Manchester vs Manchester contest to capture the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional tribute prior to the bout as ten bells tolled in remembrance of local hero Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast fight night in the city since the passing of Hatton. As the concluding sound ended, the quiet gave way to a roar of "only one Hatton".
The action during the bout matched the emotion. The eventual winner was floored by a significant blow directly to the face in the second round, rising shakily to his position on the referee's count.
But the traditional red equipment of the fighter soon advanced again. Across the ring, Rea - in opposing blue - was marked up soon, noticeable damage developing under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with effective uppercuts as the action rarely dropped over twelve rounds.
Arthur barely secured the verdict with official scores of even score, 115-113 and wider gap, but the two athletes were relentless and brave, exactly what their idol Hatton embodied.
