UK light heavyweight Buatsi secured a victory yet underwhelming debut to his career under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a split decision in a competitive 10-round fight at the Co-op Live Arena.
But the challenger landed significant shots, connecting with powerful single shots in a match where the techniques didn't complement each other, with excessive holding marring the action.
The officials judged it 96-94 for to Buatsi, with the third a even score.
"It wasn't my finest performance but how do champions respond? They prevail," the victor said in the arena as he was greeted with negative reactions from the spectators.
The Londoner got back to winning after his first professional defeat - an epic battle with his previous opponent in last winter - to claim a 20th pro success.
Various analysts thought Parker, 31, deserved the decision but the contender suffered the second defeat of his 28-fight professional journey.
Buatsi Claims Triumph in Slow Encounter
His career development has plateaued in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of UK boxing, he now stands trying to achieve expectations - possibly in what appears to be a final roll of the dice - under recent support.
He began cautiously, with both competitors willing to overcommit in a tentative opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in two years ago after breaking his hand during the bout, was competing in only his second bout at light heavyweight.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who worked behind a tight guard to narrowly win the better periods.
The spectators' acceptance wore thin amid frequent holding, with Parker slipping to the canvas several times during close exchanges.
Parker ended stronger, but several stanzas were challenging to assess. However, ex-champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the likes of hard hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester fight to capture the European light-heavyweight championship.
The arena grew hushed for a profoundly moving tribute ahead of the contest as 10 bells sounded in honor of Manchester's own Ricky Hatton, who passed away aged 46 in last month.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound ended, the quiet became a chant of "only one Hatton".
The action within the ring equaled the sentiment. The eventual winner was sent to the canvas by a left hook flush on the face in the early round, rising shakily to his feet on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. Across the ring, the opponent - in rival City blue - was injured quickly, significant bruising developing under his face as the duo exchanged powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with effective uppercuts as the pace rarely dropped over 12 rounds.
The victor narrowly won the result with judges scorecards of draw, narrow lead and 115-112, but both fighters were persistent and full of heart, all the qualities their inspiration Hatton embodied.
