British light heavyweight Buatsi secured a victory yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, connecting with powerful single shots in a match where the approaches didn't complement each other, with excessive holding marring the action.
The scoring judges judged it 96-94 in favor to the winner, with the final judge a 95-95.
"It wasn't my greatest showing but what do champions do? They win," Buatsi commented in the ringside as he was greeted with boos from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts felt Parker, thirty-one, should have been awarded the victory but the contender suffered the second defeat of his 28-fight career.
The Champion Claims Victory in Uneventful Affair
His career development has stalled in recent times through contract problems and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what seems like a final roll of the dice - under recent support.
He started carefully, with neither fighter willing to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at light-heavyweight.
By the middle rounds, the underdog had stayed in contention, creating difficulties for Buatsi, who utilized a tight guard to edge the more effective rounds.
The audience's tolerance wore thin amid constant clinching, with the challenger falling to the floor multiple occasions during clinches.
Parker ended stronger, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - working with Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian star Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Ricky Hatton Memorial Motivates All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester battle to capture the European light-heavyweight championship.
The venue became quiet for a extremely emotional memorial before the fight as ten bells rang in remembrance of Manchester's own Hatton, who passed away at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's death. As the concluding sound faded, the silence transitioned to a chant of "only one Ricky Hatton".
The competition during the bout reflected the feeling. The eventual winner was knocked down by a significant blow flush on the jaw in the early round, getting up unsteadily to his feet on the eight count.
But the red-colored equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was damaged early, noticeable damage developing under his eye as the fighters landed hard punches.
Rea, 27, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the intensity hardly diminished over twelve rounds.
The victor narrowly won the result with official scores of draw, 115-113 and clearer margin, but each competitor were determined and brave, exactly what their idol the legend embodied.
