British light heavyweight Buatsi made a winning yet disappointing beginning to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a tight ten-round bout at the boxing arena.
But the challenger found success, connecting with powerful single shots in a contest where the techniques didn't complement each other, with frequent clinching disrupting the action.
Two judges had it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best showing but what do champions do? They succeed," the victor said in the ringside as he was faced boos from the spectators.
The British fighter bounced back after his first professional defeat - an epic battle with his previous opponent in February - to earn a 20th pro success.
Various analysts thought Parker, thirty-one, deserved the decision but the Derbyshire fighter experienced the second setback of his 28-bout career.
The Champion Earns Triumph in Dull Encounter
His career development has plateaued in the past few years through management issues and considerable time of inactivity.
Once tipped as the next star of UK boxing, he now stands trying to fulfil that early promise - maybe in what appears to be a last chance - under new promotional backing.
He opened tentatively, with neither fighter willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against John Ryder in 2022 after suffering a hand injury in the contest, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for the favorite, who utilized a solid defense to edge the better periods.
The audience's tolerance diminished because of frequent holding, with Parker slipping to the canvas multiple occasions during grappling.
The underdog ended stronger, but many rounds were hard to judge. Although, former world champion Carl Frampton - providing analysis for the broadcast network - had it strongly leaning for the challenger, giving Buatsi just one round.
Buatsi progresses, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Hatton Tribute Motivates Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Rea by divided decision in a excellent Manchester vs Manchester fight to secure the European light heavyweight championship.
The arena fell silent for a profoundly moving memorial ahead of the contest as ten bells tolled in remembrance of local hero Hatton, who died at age 46 in last month.
It was the premier televised fight night in the location since Hatton's death. As the final chime faded, the silence transitioned to a roar of "there's just one Ricky Hatton".
The action during the bout matched the sentiment. Arthur was knocked down by a left hook squarely on the chin in the initial stage, getting up unsteadily to his position on the count of eight.
But the red-colored attire of Arthur soon came forward again. On the opposite side, the opponent - in opposing blue - was injured early, noticeable damage forming under his face as the fighters traded hard punches.
Rea, 27, landed power punches but Arthur, 34, fought fire with fire with powerful upward punches as the pace rarely dropped over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, 115-113 and 115-112, but the two athletes were relentless and full of heart, exactly what their inspiration the legend stood for.
