UK light-heavyweight Buatsi made a winning yet unimpressive start to his career under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a judges' verdict in a tight 10-round bout at the Co-op Live Arena.
But the challenger had his moments, delivering effective individual punches in a match where the styles never quite clicked, with constant grabbing disrupting the fight.
The scoring judges scored it 96-94 in favor to the winner, with the final judge a 95-95 draw.
"It wasn't my finest display but how do champions respond? They succeed," Buatsi commented in the ring as he was greeted with boos from the audience.
The Londoner got back to winning after his maiden career setback - an memorable fight with his previous opponent in February - to claim a 20th pro success.
Some ringside observers thought Parker, 31, deserved the win but the Derbyshire fighter endured the second defeat of his 28-bout career.
The Champion Claims Victory in Slow Affair
Buatsi's progress has plateaued in the past few years through promotional disputes and considerable time of lack of fights.
Previously predicted as the next star of the British fight scene, he now is positioned trying to realize his potential - perhaps in what feels like a final roll of the dice - under fresh management.
He began cautiously, with the two boxers ready to take risks in a calculated initial rounds.
Parker, whose only previous defeat came at the lower weight class against John Ryder in two years ago after breaking his hand in the contest, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had made it competitive, making life awkward for Buatsi, who operated with a solid defense to narrowly win the better periods.
The spectators' acceptance diminished during constant clinching, with the challenger falling to the floor several times during clinches.
Parker ended stronger, but several stanzas were challenging to assess. However, former world champion the boxing analyst - working with Dazn - had it strongly leaning for the challenger, scoring for Buatsi just one round.
Buatsi progresses, but with the likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still appears challenging.
Ricky Hatton Memorial Inspires Manchester Derby Fight
In the secondary fight, Lyndon Arthur overcame Bradley Rea by close verdict in a excellent local rivalry fight to capture the continental belt.
The venue fell silent for a extremely emotional tribute ahead of the contest as a bell ceremony sounded in remembrance of Manchester's own Ricky Hatton, who succumbed at age 46 in recently.
It was the premier televised boxing event in the location since Hatton's death. As the concluding sound ended, the stillness gave way to a roar of "only one Ricky Hatton".
The competition inside the ropes equaled the feeling. The eventual winner was sent to the canvas by a significant blow directly to the jaw in the initial stage, rising shakily to his feet on the eight count.
But the red-colored equipment of the fighter soon came forward again. From the other corner, the opponent - in rival City blue - was injured early, heavy swelling appearing under his eye as the duo exchanged powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the action rarely dropped over twelve rounds.
Arthur edged the verdict with official scores of 114-114, close margin and clearer margin, but each competitor were persistent and full of heart, everything their idol Hatton represented.
