UK light heavyweight Buatsi earned a win yet underwhelming start to his career under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a judges' verdict in a competitive 10-round fight at the boxing arena.
But Parker had his moments, delivering effective clean blows in a fight where the approaches didn't complement each other, with constant grabbing spoiling the fight.
The scoring judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest showing but what defines champions? They prevail," Buatsi commented in the arena as he was met by negative reactions from the spectators.
The British fighter returned to winning ways after his maiden career setback - an memorable fight with his previous opponent in February - to secure a 20th pro success.
Several boxing experts believed the challenger, thirty-one, merited the win but the contender suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Encounter
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to achieve expectations - possibly in what seems like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a calculated early stages.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in 2022 after breaking his hand in the contest, was making just his second fight at the higher weight class.
By the halfway stage, Parker had made it competitive, causing problems for Buatsi, who worked behind a tight guard to barely take the cleaner rounds.
The crowd's patience decreased amid constant clinching, with the challenger going down several times during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. However, former world champion Carl Frampton - providing analysis for Dazn - had it significantly in favor for Parker, scoring for Buatsi just a single round.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still appears challenging.
Ricky Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Arthur defeated Rea by close verdict in a excellent all-Manchester fight to capture the European light heavyweight title.
The stadium grew hushed for a profoundly moving homage prior to the bout as ten bells rang in remembrance of the city's native Ricky Hatton, who succumbed 46 years old in September.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the last bell disappeared, the quiet gave way to a roar of "only one Hatton".
The action during the bout equaled the emotion. The eventual winner was sent to the canvas by a left hook flush on the chin in the second round, getting up unsteadily to his feet on the referee's count.
But the Manchester United-red attire of the fighter soon came forward again. Across the ring, Rea - in competitive blue - was marked up soon, heavy swelling appearing under his eye as the pair landed hard punches.
Rea, twenty-seven, delivered effective strikes but the champion, thirty-four, matched the intensity with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor edged the result with official scores of draw, 115-113 and wider gap, but the two athletes were determined and brave, everything their inspiration Hatton represented.
