UK light-heavyweight Buatsi secured a victory yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's work rate likely gained him a split decision in a close 10-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a fight where the approaches failed to mesh, with constant grabbing disrupting the fight.
The officials judged it 96-94 in favor to the winner, with the third a even score.
"It wasn't my greatest showing but what defines champions? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the spectators.
The Londoner returned to winning ways after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the British boxer endured the second defeat of his 28-fight career.
Buatsi Earns Victory in Uneventful Match
His career development has slowed in recent years through management issues and extended periods of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He started carefully, with the two boxers willing to be aggressive in a tentative early stages.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second bout at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, making life awkward for the favorite, who operated with a tight guard to barely take the better stanzas.
The audience's tolerance diminished during regular grabbing, with the challenger falling to the floor several times during clinches.
The underdog finished the stronger, but several stanzas were hard to judge. However, ex-champion the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, awarding Buatsi just only one stanza.
The victor advances, but with the likes of power puncher David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Memorial Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by close verdict in a terrific Manchester vs Manchester fight to capture the continental title.
The venue grew hushed for a deeply poignant homage prior to the bout as a bell ceremony sounded in remembrance of local hero Ricky Hatton, who passed away aged 46 in September.
It was the first major televised boxing event in the location since the passing of Hatton. As the concluding sound faded, the stillness became a roar of "there's only one Hatton".
The fighting during the bout matched the emotion. The eventual winner was floored by a left hook squarely on the chin in the initial stage, rising shakily to his position on the count of eight.
But the Manchester United-red equipment of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged early, significant bruising forming under his face as the duo traded punishing blows.
The challenger, 27, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over 12 rounds.
The victor narrowly won the decision with scoring tallies of even score, 115-113 and clearer margin, but the two athletes were determined and brave, all the qualities their inspiration the legend represented.
