UK light-heavyweight Buatsi made a winning yet underwhelming beginning to his career under recent management with a points victory over Parker in Manchester.
The Olympic medal winner's activity level likely gained him a split decision in a close ten-round contest at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective clean blows in a match where the techniques didn't complement each other, with excessive holding spoiling the action.
The officials scored it 96-94 to the winner, with the third a 95-95.
"It wasn't my best showing but how do champions respond? They succeed," Buatsi said in the arena as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his first professional defeat - an thrilling contest with Callum Smith in February - to earn a 20th pro success.
Several boxing experts felt Parker, 31, deserved the decision but the contender endured the second defeat of his 28-match career.
Buatsi Claims Triumph in Slow Affair
Buatsi's progress has plateaued in recent times through management issues and considerable time of inactivity.
Once tipped as the future of UK boxing, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He started carefully, with both competitors prepared to be aggressive in a calculated opening few rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was making just his second fight at light-heavyweight.
By the halfway stage, Parker had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to edge the more effective periods.
The audience's tolerance diminished amid constant clinching, with the challenger falling to the floor various instances during close exchanges.
The underdog ended stronger, but multiple periods were difficult to score. However, former world champion the boxing analyst - providing analysis for Dazn - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor advances, but with the caliber of hard hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Hatton Homage Fuels All-Manchester Battle
In the supporting bout, Arthur defeated Bradley Rea by split-decision in a outstanding local rivalry fight to secure the European light-heavyweight belt.
The arena fell silent for a profoundly moving homage ahead of the contest as 10 bells rang in honor of Manchester's own Ricky Hatton, who succumbed 46 years old in September.
It was the first major televised boxing event in the location since the passing of Hatton. As the last bell disappeared, the stillness gave way to a chant of "only one Hatton".
The fighting inside the ropes matched the feeling. Arthur was sent to the canvas by a significant blow squarely on the chin in the early round, rising shakily to his position on the eight count.
But the red-colored equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged soon, noticeable damage developing under his orbital area as the fighters traded powerful shots.
The challenger, 27, connected with strong blows but the champion, 34, responded in kind with powerful upward punches as the intensity seldom decreased over 12 rounds.
Arthur narrowly won the verdict with official scores of even score, 115-113 and 115-112, but both fighters were determined and brave, everything their hero Hatton stood for.
