UK light-heavyweight Buatsi secured a victory yet underwhelming debut to his career under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a close ten-round fight at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a match where the styles never quite clicked, with excessive holding marring the fight.
The officials judged it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my greatest display but what do champions do? They succeed," the victor said in the arena as he was met by disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to claim a 20th pro success.
Various analysts believed the challenger, 31, deserved the win but the British boxer endured the second setback of his 28-fight professional journey.
The Champion Secures Win in Uneventful Match
Buatsi's progress has stalled in recent years through contract problems and long spells of inactivity.
Earlier forecast as the future of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what feels like a final roll of the dice - under fresh management.
He started carefully, with neither fighter willing to take risks in a cagey initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand during the bout, was competing in only his second bout at light-heavyweight.
By the halfway stage, Parker had kept it close, creating difficulties for the favorite, who utilized a protective stance to barely take the better stanzas.
The spectators' acceptance wore thin because of constant clinching, with the challenger going down multiple occasions during grappling.
Parker ended stronger, but multiple periods were difficult to score. Nevertheless, ex-champion Carl Frampton - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur defeated Rea by divided decision in a outstanding Manchester vs Manchester contest to win the European light heavyweight belt.
The stadium fell silent for a profoundly moving memorial ahead of the contest as ten bells tolled in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the first major televised match card in the urban center since Hatton's passing. As the concluding sound ended, the stillness transitioned to a roar of "there's just one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was floored by a left hook directly to the face in the early round, standing uncertainly to his position on the count of eight.
But the traditional red equipment of Arthur soon came forward again. On the opposite side, the opponent - in competitive blue - was marked up soon, heavy swelling developing under his eye as the duo exchanged punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with powerful upward punches as the action hardly diminished over the full distance.
Arthur narrowly won the verdict with judges scorecards of 114-114, 115-113 and wider gap, but both fighters were persistent and full of heart, all the qualities their hero the legend represented.
