UK light-heavyweight Buatsi earned a win yet underwhelming start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic medal winner's work rate likely gained him a split decision in a competitive ten-round fight at the Co-op Live Arena.
But the challenger found success, delivering effective clean blows in a match where the styles failed to mesh, with constant grabbing marring the fight.
The officials judged it 96-94 to Buatsi, with the remaining official a even score.
"It wasn't my best performance but how do champions respond? They win," Buatsi commented in the ringside as he was faced negative reactions from the spectators.
The British fighter returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to earn a 20th career victory.
Various analysts thought Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second loss of his 28-fight professional journey.
The Champion Earns Victory in Dull Match
His career development has slowed in the past few years through management issues and extended periods of lack of fights.
Previously predicted as the next star of the British fight scene, he now finds himself trying to fulfil that early promise - perhaps in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with both competitors ready to overcommit in a tentative early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand in the contest, was participating in his second fight at light heavyweight.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who operated with a solid defense to narrowly win the better stanzas.
The audience's tolerance decreased amid constant clinching, with Parker falling to the floor multiple occasions during clinches.
The underdog ended stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it particularly wide for Parker, scoring for Buatsi just one round.
The victor advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol leading the category, the journey to titles - based on this performance - still seems difficult.
Ricky Hatton Memorial Inspires All-Manchester Battle
In the secondary fight, Lyndon Arthur defeated Bradley Rea by split-decision in a terrific Manchester vs Manchester fight to win the continental title.
The arena grew hushed for a extremely emotional memorial ahead of the contest as a bell ceremony tolled in memory of Manchester's own Hatton, who died aged 46 in recently.
It was the premier televised match card in the city since Hatton's death. As the last bell ended, the stillness became a chant of "there's only one Hatton".
The competition within the ring equaled the emotion. Arthur was sent to the canvas by a significant blow squarely on the jaw in the second round, rising shakily to his feet on the referee's count.
But the red-colored attire of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was damaged early, heavy swelling developing under his face as the duo landed powerful shots.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, fought fire with fire with effective uppercuts as the action rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of even score, narrow lead and wider gap, but both fighters were determined and full of heart, exactly what their hero the legend embodied.
