British light-heavyweight Buatsi made a winning yet unimpressive beginning to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a judges' verdict in a tight ten-round fight at the boxing arena.
But the challenger landed significant shots, delivering effective clean blows in a contest where the techniques didn't complement each other, with excessive holding disrupting the action.
The officials had it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my finest performance but what do champions do? They win," Buatsi commented in the ring as he was greeted with boos from the spectators.
The Londoner got back to winning after his first professional defeat - an memorable fight with Callum Smith in last winter - to earn a twentieth professional win.
Several boxing experts believed the challenger, 31, should have been awarded the victory but the British boxer endured the second setback of his 28-bout career.
Buatsi Secures Victory in Dull Affair
The fighter's advancement has stalled in the past few years through promotional disputes and long spells of lack of fights.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a ultimate opportunity - under recent support.
He began cautiously, with the two boxers prepared to take risks in a cagey early stages.
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 in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, the underdog had kept it close, causing problems for Buatsi, who utilized a protective stance to barely take the more effective stanzas.
The crowd's patience decreased amid frequent holding, with the challenger going down various instances during close exchanges.
Parker closed well, but many rounds were hard to judge. Although, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, awarding Buatsi just a single round.
The victor advances, but with the caliber of power puncher David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - on this evidence - still looks steep.
Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding Manchester vs Manchester fight to secure the European light heavyweight belt.
The venue fell silent for a extremely emotional tribute before the fight as ten bells rang in honor of local hero Ricky Hatton, who died aged 46 in September.
It was the initial significant broadcast fight night in the urban center since Hatton's death. As the last bell faded, the silence transitioned to a shout of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. Arthur was knocked down by a powerful punch directly to the face in the initial stage, standing uncertainly to his feet on the eight count.
But the Manchester United-red shorts of the fighter soon pressed ahead again. From the other corner, Rea - in opposing blue - was marked up soon, significant bruising appearing under his face as the pair traded punishing blows.
The challenger, 27, connected with strong blows but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over the full distance.
Arthur narrowly won the decision with official scores of draw, narrow lead and 115-112, but both fighters were persistent and full of heart, all the qualities their hero Hatton embodied.
