UK light-heavyweight Buatsi secured a victory yet disappointing debut to life under new promoters with a unanimous decision over Parker in the Manchester arena.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive 10-round bout at the boxing arena.
But Parker found success, delivering effective single shots in a fight where the styles failed to mesh, with excessive holding disrupting the contest.
Two judges had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my greatest display but how do champions respond? They succeed," the victor said in the arena as he was faced disapproval from the crowd.
The British fighter got back to winning after his initial pro loss - an epic battle with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts believed the challenger, thirty-one, merited the victory but the British boxer suffered the second setback of his 28-bout professional journey.
The Champion Secures Win in Dull Match
Buatsi's progress has plateaued in recent years through management issues and considerable time of inactivity.
Previously predicted as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what feels like a last chance - under fresh management.
He began cautiously, with the two boxers willing to be aggressive in a cagey opening few rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand in the contest, was competing in only his second bout at light heavyweight.
By the fight's midpoint, Parker had kept it close, creating difficulties for Buatsi, who utilized a tight guard to narrowly win the better periods.
The crowd's patience decreased during regular grabbing, with the challenger going down several times during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, former world champion the boxing analyst - working with Dazn - had it significantly in favor for the challenger, scoring for Buatsi just one round.
Buatsi advances, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Homage Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur overcame Bradley Rea by split-decision in a terrific all-Manchester fight to win the continental championship.
The venue became quiet for a deeply poignant homage before the fight as ten bells tolled in remembrance of local hero Hatton, who died 46 years old in last month.
It was the premier televised match card in the location since Hatton's death. As the concluding sound faded, the silence became a shout of "there's only one Hatton".
The action inside the ropes reflected the emotion. Arthur was sent to the canvas by a powerful punch flush on the face in the second round, standing uncertainly to his feet on the referee's count.
But the red-colored attire of the fighter soon came forward again. On the opposite side, Rea - in competitive blue - was damaged early, significant bruising appearing under his eye as the fighters landed powerful shots.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the intensity rarely dropped over the full distance.
Arthur barely secured the verdict with judges scorecards of even score, 115-113 and 115-112, but both fighters were relentless and full of heart, all the qualities their inspiration Hatton represented.
