UK light-heavyweight Buatsi earned a win yet disappointing beginning to life under recent management with a decision win over Parker in Manchester.
The Olympic bronze medallist's work rate likely gained him a majority decision in a competitive ten-round contest at the Manchester venue.
But the challenger had his moments, landing some telling clean blows in a contest where the techniques didn't complement each other, with frequent clinching marring the contest.
The scoring judges judged it 96-94 for to the winner, with the third a 95-95.
"It wasn't my greatest display but what do champions do? They prevail," the victor commented in the ringside as he was faced disapproval from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with Callum Smith in February - to earn a 20th career victory.
Several boxing experts believed Parker, thirty-one, deserved the victory but the contender suffered the second setback of his 28-bout professional journey.
Buatsi Earns Victory in Dull Encounter
His career development has stalled in recent times through promotional disputes and considerable time of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - maybe in what appears to be a final roll of the dice - under new promotional backing.
He began cautiously, with neither fighter prepared to take risks in a tentative initial rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand during the bout, was participating in his second bout at the higher weight class.
By the halfway stage, Parker had stayed in contention, causing problems for the favorite, who utilized a protective stance to edge the better periods.
The audience's tolerance wore thin during constant clinching, with the challenger falling to the floor several times during clinches.
Parker closed well, but multiple periods were difficult to score. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it significantly in favor for the challenger, scoring for Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - based on this performance - still looks steep.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the secondary fight, Arthur defeated Rea by close verdict in a excellent Manchester vs Manchester fight to win the European light-heavyweight belt.
The venue became quiet for a profoundly moving memorial ahead of the contest as 10 bells sounded in memory of local hero Ricky Hatton, who died 46 years old in September.
It was the first major televised boxing event in the location since Hatton's passing. As the last bell ended, the silence transitioned to a shout of "there's only one Hatton".
The fighting during the bout reflected the feeling. The eventual winner was sent to the canvas by a significant blow flush on the jaw in the early round, standing uncertainly to his feet on the count of eight.
But the traditional red equipment of Arthur soon pressed ahead again. On the opposite side, the opponent - in rival City blue - was injured soon, significant bruising forming under his eye as the duo traded hard punches.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the action hardly diminished over twelve rounds.
The victor edged the verdict with scoring tallies of even score, 115-113 and 115-112, but both fighters were relentless and courageous, everything their inspiration Hatton stood for.
