British light-heavyweight Buatsi secured a victory yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a split decision in a close ten-round fight at the Manchester venue.
But Parker landed significant shots, delivering effective individual punches in a match where the styles never quite clicked, with frequent clinching spoiling the action.
The scoring judges had it 96-94 to the winner, with the third a even score.
"It wasn't my best display but how do champions respond? They win," Buatsi stated in the ringside as he was faced boos from the spectators.
The British fighter returned to winning ways after his first professional defeat - an thrilling contest with his previous opponent in February - to secure a twentieth professional win.
Various analysts thought the challenger, 31, should have been awarded the decision but the Derbyshire fighter experienced the second loss of his 28-bout professional journey.
Buatsi Earns Triumph in Slow Match
The fighter's advancement has stalled in recent years through contract problems and long spells of lack of fights.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - possibly in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with neither fighter willing to overcommit in a tentative early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in two years ago after damaging his hand mid-fight, was competing in only his second fight at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who utilized a solid defense to narrowly win the better periods.
The crowd's patience decreased amid regular grabbing, with the challenger slipping to the canvas various instances during clinches.
Parker finished the stronger, but multiple periods were challenging to assess. However, previous titleholder Carl Frampton - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just one round.
The victor moves on, but with the caliber of hard hitter David Benavidez, and Russian star Dmitry Bivol dominating the weight class, the journey to titles - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a excellent all-Manchester fight to capture the continental belt.
The venue fell silent for a extremely emotional tribute prior to the bout as ten bells sounded in honor of local hero Ricky Hatton, who died at age 46 in September.
It was the first major televised match card in the location since Hatton's death. As the last bell ended, the quiet transitioned to a roar of "there's just one Hatton".
The action inside the ropes equaled the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was injured soon, heavy swelling appearing under his face as the fighters traded hard punches.
The challenger, 27, delivered effective strikes but the champion, 34, matched the intensity with effective uppercuts as the pace hardly diminished over twelve rounds.
Arthur barely secured the verdict with judges scorecards of 114-114, 115-113 and wider gap, but the two athletes were determined and brave, all the qualities their hero Hatton embodied.
