British light-heavyweight Buatsi secured a victory yet underwhelming start to life under recent management with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's punch output appeared to earn him a majority decision in a close ten-round bout at the Co-op Live Arena.
But Parker landed significant shots, landing some telling single shots in a fight where the styles didn't complement each other, with excessive holding spoiling the contest.
Two judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my greatest display but what do champions do? They succeed," Buatsi said in the ringside as he was met by negative reactions from the crowd.
The British fighter got back to winning after his first professional defeat - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers felt Parker, 31, should have been awarded the victory but the Derbyshire fighter suffered the second setback of his 28-match career.
Buatsi Earns Win in Slow Affair
The fighter's advancement has plateaued in the past few years through contract problems and considerable time of inactivity.
Earlier forecast as the rising talent of UK boxing, he now finds himself trying to realize his potential - maybe in what feels like a ultimate opportunity - under new promotional backing.
He began cautiously, with neither fighter prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after damaging his hand mid-fight, was participating in his second appearance at light-heavyweight.
By the fight's midpoint, Parker had stayed in contention, creating difficulties for Buatsi, who utilized a solid defense to edge the more effective stanzas.
The audience's tolerance diminished during frequent holding, with Parker going down multiple occasions during grappling.
Parker finished the stronger, but many rounds were hard to judge. Although, previous titleholder Carl Frampton - working with Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
Buatsi moves on, but with the presence of hard hitter David Benavidez, and Eastern European fighter Bivol leading the category, the path to championship success - based on this performance - still seems difficult.
Hatton Tribute Fuels All-Manchester Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a outstanding Manchester vs Manchester battle to secure the European light-heavyweight championship.
The stadium grew hushed for a extremely emotional tribute before the fight as a bell ceremony sounded in memory of local hero Hatton, who died aged 46 in recently.
It was the premier televised boxing event in the location since Hatton's death. As the final chime ended, the quiet gave way to a chant of "there's just one Ricky Hatton".
The competition within the ring equaled the sentiment. Arthur was floored by a left hook flush on the face in the initial stage, rising shakily to his footing on the referee's count.
But the red-colored equipment of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was damaged quickly, significant bruising developing under his face as the duo landed powerful shots.
The challenger, twenty-seven, connected with strong blows but the champion, 34, matched the intensity with stinging uppercuts as the action hardly diminished over 12 rounds.
Arthur barely secured the result with judges scorecards of even score, narrow lead and wider gap, but both fighters were relentless and full of heart, all the qualities their hero the legend represented.
