British light-heavyweight Buatsi secured a victory yet unimpressive beginning to life under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output likely gained him a judges' verdict in a close ten-round fight at the Co-op Live Arena.
But Parker landed significant shots, delivering effective individual punches in a match where the approaches didn't complement each other, with excessive holding disrupting the action.
The officials judged it 96-94 to the winner, with the final judge a even score.
"It wasn't my greatest display but what defines champions? They prevail," Buatsi said in the ring as he was met by boos from the crowd.
The British fighter got back to winning after his first professional defeat - an thrilling contest with his previous opponent in last winter - to claim a 20th career victory.
Several boxing experts believed the challenger, thirty-one, should have been awarded the decision but the contender endured the second defeat of his 28-bout professional journey.
The Champion Claims Win in Dull Match
His career development has stalled in recent times through promotional disputes and extended periods of inactivity.
Previously predicted as the next star of UK boxing, he now finds himself trying to realize his potential - perhaps in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to take risks in a tentative early stages.
Parker, whose only previous defeat came at super middleweight against John Ryder in two years ago after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the fight's midpoint, the underdog had made it competitive, making life awkward for Buatsi, who worked behind a protective stance to barely take the cleaner periods.
The spectators' acceptance decreased amid regular grabbing, with Parker slipping to the canvas various instances during grappling.
The underdog ended stronger, but multiple periods were hard to judge. Although, former world champion Carl Frampton - working with Dazn - had it significantly in favor for the challenger, giving Buatsi just one round.
Buatsi moves on, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the path to championship success - based on this performance - still seems difficult.
Ricky Hatton Tribute Motivates All-Manchester Battle
In the co-main event, Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester contest to capture the European light heavyweight belt.
The arena fell silent for a deeply poignant homage before the fight as 10 bells sounded in remembrance of local hero Hatton, who died 46 years old in September.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound disappeared, the silence became a chant of "there's only one Hatton".
The action within the ring matched the feeling. The eventual winner was floored by a significant blow flush on the face in the early round, rising shakily to his position on the eight count.
But the red-colored shorts of the fighter soon pressed ahead again. From the other corner, Rea - in competitive blue - was marked up soon, noticeable damage appearing under his orbital area as the duo landed hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, responded in kind with effective uppercuts as the action rarely dropped over the full distance.
The victor edged the result with scoring tallies of even score, 115-113 and wider gap, but each competitor were determined and brave, everything their hero Hatton represented.
