UK light heavyweight Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output appeared to earn him a split decision in a tight 10-round contest at the Co-op Live Arena.
But the challenger had his moments, connecting with powerful individual punches in a fight where the techniques didn't complement each other, with excessive holding spoiling the action.
Two judges scored it 96-94 for to the winner, with the remaining official a 95-95 draw.
"It wasn't my greatest showing but how do champions respond? They succeed," the victor said in the ring as he was greeted with disapproval from the crowd.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Some ringside observers believed the challenger, thirty-one, merited the win but the Derbyshire fighter experienced the second loss of his 28-match career.
Buatsi Secures Win in Slow Encounter
His career development has slowed in recent years through contract problems and extended periods of inactivity.
Once tipped as the future of British boxing, he now finds himself trying to achieve expectations - possibly in what appears to be a last chance - under recent support.
He began cautiously, with both competitors prepared to take risks in a tentative early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand during the bout, was making just his second fight at the higher weight class.
By the fight's midpoint, Parker had made it competitive, causing problems for Buatsi, who utilized a protective stance to narrowly win the better stanzas.
The crowd's patience diminished amid constant clinching, with Parker slipping to the canvas several times during clinches.
The underdog ended stronger, but multiple periods were hard to judge. Nevertheless, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
The victor advances, but with the likes of heavy hitter Benavidez, and Eastern European fighter Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Tribute Motivates Manchester Derby Fight
In the secondary fight, Lyndon Arthur beat Rea by close verdict in a terrific local rivalry battle to capture the European light-heavyweight title.
The venue grew hushed for a profoundly moving tribute ahead of the contest as a bell ceremony tolled in honor of Manchester's own Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the city since Hatton's death. As the final chime faded, the silence gave way to a shout of "only one Hatton".
The fighting inside the ropes equaled the emotion. The eventual winner was floored by a significant blow squarely on the chin in the second round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red shorts of Arthur soon advanced again. From the other corner, the opponent - in rival City blue - was injured early, significant bruising appearing under his orbital area as the pair exchanged hard punches.
Rea, twenty-seven, connected with strong blows but Arthur, 34, responded in kind with stinging uppercuts as the intensity seldom decreased over twelve rounds.
Arthur narrowly won the verdict with judges scorecards of even score, 115-113 and clearer margin, but both fighters were determined and full of heart, exactly what their idol Hatton represented.
