UK light-heavyweight Buatsi made a winning yet unimpressive debut to life under recent management with a decision win over Parker in Manchester.
The Olympic medal winner's activity level seemingly secured him a majority decision in a close 10-round contest at the Co-op Live Arena.
But the challenger found success, delivering effective individual punches in a contest where the approaches didn't complement each other, with excessive holding marring the action.
The officials had it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my finest display but what do champions do? They prevail," Buatsi stated in the ring as he was met by disapproval from the crowd.
The Londoner returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to claim a twentieth professional win.
Some ringside observers thought the challenger, 31, should have been awarded the decision but the Derbyshire fighter suffered the second setback of his 28-fight career.
Buatsi Secures Win in Dull Encounter
Buatsi's progress has slowed in the past few years through promotional disputes and extended periods of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to achieve expectations - maybe in what appears to be a ultimate opportunity - under recent support.
He began cautiously, with the two boxers willing to be aggressive in a cagey opening few rounds.
The challenger, whose sole prior loss came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was participating in his second appearance at the higher weight class.
By the halfway stage, the underdog had stayed in contention, causing problems for the favorite, who utilized a solid defense to edge the more effective periods.
The audience's tolerance wore thin because of regular grabbing, with the challenger going down multiple occasions during close exchanges.
Parker finished the stronger, but many rounds were hard to judge. However, previous titleholder the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, giving Buatsi just one round.
The victor progresses, but with the presence of heavy hitter David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Memorial Inspires All-Manchester Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a excellent local rivalry fight to capture the European light-heavyweight belt.
The stadium grew hushed for a profoundly moving tribute before the fight as ten bells rang in remembrance of the city's native Ricky Hatton, who died at age 46 in September.
It was the first major televised boxing event in the urban center since Hatton's passing. As the final chime faded, the stillness gave way to a shout of "only one Hatton".
The action during the bout reflected the emotion. Arthur was floored by a powerful punch squarely on the jaw in the second round, getting up unsteadily to his footing on the count of eight.
But the traditional red shorts of the fighter soon pressed ahead again. Across the ring, Rea - in competitive blue - was damaged soon, significant bruising appearing under his face as the duo landed punishing blows.
The challenger, twenty-seven, landed power punches but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over the full distance.
Arthur narrowly won the result with official scores of 114-114, 115-113 and clearer margin, but the two athletes were determined and courageous, exactly what their hero the legend represented.
