UK light heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate likely gained him a split decision in a tight 10-round fight at the boxing arena.
But Parker landed significant shots, connecting with powerful clean blows in a match where the styles never quite clicked, with constant grabbing disrupting the action.
The scoring judges judged it 96-94 for to the winner, with the final judge a 95-95.
"It wasn't my best showing but what do champions do? They prevail," the victor stated in the ring as he was met by disapproval from the crowd.
The British fighter bounced back after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Various analysts believed Parker, 31, merited the victory but the Derbyshire fighter endured the second loss of his 28-bout professional journey.
Buatsi Secures Triumph in Dull Encounter
The fighter's advancement has slowed in recent years through contract problems and considerable time of inactivity.
Once tipped as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter ready to be aggressive in a calculated early stages.
Parker, whose single earlier setback came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was participating in his second appearance at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a solid defense to narrowly win the cleaner periods.
The spectators' acceptance wore thin during frequent holding, with Parker slipping to the canvas various instances during close exchanges.
The underdog ended stronger, but several stanzas were difficult to score. Although, ex-champion the boxing analyst - working with Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
The victor moves on, but with the presence of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Fuels Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Rea by close verdict in a outstanding all-Manchester fight to secure the European light-heavyweight championship.
The arena fell silent for a extremely emotional tribute before the fight as 10 bells tolled in honor of the city's native Ricky Hatton, who succumbed 46 years old in last month.
It was the premier televised fight night in the city since Hatton's death. As the final chime disappeared, the silence transitioned to a chant of "there's only one Hatton".
The competition within the ring reflected the sentiment. The eventual winner was floored by a significant blow directly to the chin in the initial stage, standing uncertainly to his feet on the eight count.
But the traditional red attire of Arthur soon pressed ahead again. From the other corner, Rea - in rival City blue - was damaged soon, noticeable damage appearing under his face as the fighters traded hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity hardly diminished over twelve rounds.
The victor narrowly won the decision with scoring tallies of even score, close margin and 115-112, but both fighters were persistent and courageous, everything their hero Hatton embodied.
