British light-heavyweight Buatsi made a winning yet underwhelming debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a tight ten-round fight at the Co-op Live Arena.
But Parker found success, landing some telling single shots in a match where the styles never quite clicked, with excessive holding disrupting the contest.
The scoring judges had it 96-94 to Buatsi, with the remaining official a 95-95 draw.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi commented in the ringside as he was faced boos from the crowd.
The British fighter bounced back after his maiden career setback - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts believed the challenger, 31, should have been awarded the decision but the contender experienced the second defeat of his 28-bout professional journey.
The Champion Claims Triumph in Uneventful Encounter
The fighter's advancement has slowed in the past few years through promotional disputes and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what seems like a ultimate opportunity - under new promotional backing.
He started carefully, with the two boxers prepared to take risks in a cagey early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand mid-fight, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had stayed in contention, making life awkward for the favorite, who operated with a solid defense to barely take the better rounds.
The audience's tolerance decreased because of regular grabbing, with Parker going down multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were difficult to score. Although, former world champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just a single round.
The victor advances, but with the caliber of power puncher David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - on this evidence - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Rea by divided decision in a excellent all-Manchester fight to capture the European light-heavyweight championship.
The stadium fell silent for a extremely emotional homage before the fight as a bell ceremony tolled in memory of Manchester's own Hatton, who succumbed aged 46 in recently.
It was the first major televised match card in the location since the passing of Hatton. As the concluding sound faded, the silence gave way to a shout of "only one Ricky Hatton".
The competition within the ring matched the sentiment. Arthur was floored by a left hook flush on the chin in the initial stage, rising shakily to his footing on the count of eight.
But the Manchester United-red shorts of the fighter soon advanced again. On the opposite side, the opponent - in opposing blue - was damaged soon, significant bruising forming under his eye as the pair landed punishing blows.
Rea, twenty-seven, connected with strong blows but Arthur, thirty-four, matched the intensity with effective uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the result with judges scorecards of 114-114, narrow lead and 115-112, but both fighters were determined and full of heart, everything their inspiration the legend stood for.
