British light-heavyweight Buatsi secured a victory yet underwhelming debut to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic medal winner's punch output likely gained him a majority decision in a competitive 10-round fight at the boxing arena.
But the challenger found success, connecting with powerful single shots in a match where the styles never quite clicked, with excessive holding disrupting the fight.
Two judges judged it 96-94 for to Buatsi, with the third a 95-95 draw.
"It wasn't my best display but how do champions respond? They prevail," Buatsi stated in the ring as he was faced disapproval from the spectators.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in February - to secure a twentieth professional win.
Various analysts believed Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter experienced the second setback of his 28-fight career.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has slowed in recent times through promotional disputes and long spells of lack of fights.
Once tipped as the rising talent of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He started carefully, with both competitors prepared to be aggressive in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was participating in his second fight at light-heavyweight.
By the halfway stage, the underdog had kept it close, making life awkward for the favorite, who utilized a solid defense to barely take the more effective stanzas.
The audience's tolerance wore thin during regular grabbing, with Parker slipping to the canvas multiple occasions during close exchanges.
Parker finished the stronger, but several stanzas were hard to judge. Nevertheless, ex-champion Carl Frampton - commentating on the broadcast network - had it particularly wide for Parker, awarding Buatsi just one round.
The victor moves on, but with the presence of heavy hitter Benavidez, and Eastern European fighter Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur defeated Rea by split-decision in a terrific all-Manchester fight to capture the continental title.
The stadium became quiet for a deeply poignant tribute prior to the bout as a bell ceremony rang in honor of local hero Hatton, who succumbed aged 46 in recently.
It was the premier televised boxing event in the city since Hatton's death. As the last bell ended, the stillness became a shout of "there's just one Hatton".
The competition inside the ropes reflected the sentiment. Arthur was knocked down by a significant blow flush on the jaw in the second round, standing uncertainly to his feet on the referee's count.
But the traditional red shorts of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was injured quickly, noticeable damage appearing under his face as the pair exchanged hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, matched the intensity with powerful upward punches as the pace hardly diminished over the full distance.
The victor barely secured the result with judges scorecards of even score, close margin and 115-112, but the two athletes were determined and brave, all the qualities their inspiration the legend stood for.
