British light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's activity level seemingly secured him a judges' verdict in a competitive ten-round bout at the Manchester venue.
But the challenger found success, landing some telling individual punches in a contest where the styles didn't complement each other, with constant grabbing disrupting the fight.
Two judges judged it 96-94 to the winner, with the remaining official a 95-95.
"It wasn't my best performance but how do champions respond? They win," Buatsi said in the ringside as he was met by disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th pro success.
Several boxing experts believed the challenger, thirty-one, merited the decision but the British boxer experienced the second defeat of his 28-fight professional journey.
The Champion Secures Win in Slow Encounter
His career development has slowed in the past few years through promotional disputes and long spells of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to realize his potential - possibly in what seems like a ultimate opportunity - under fresh management.
He began cautiously, with neither fighter prepared to overcommit in a calculated opening few rounds.
The challenger, whose only previous defeat came at the lower weight class against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second fight at light heavyweight.
By the halfway stage, Parker had made it competitive, creating difficulties for Buatsi, who operated with a solid defense to narrowly win the cleaner periods.
The audience's tolerance diminished because of frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but several stanzas were hard to judge. Nevertheless, previous titleholder Carl Frampton - working with the broadcast network - had it particularly wide for the challenger, awarding Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher David Benavidez, and Russian champion Dmitry Bivol dominating the weight class, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the supporting bout, Arthur overcame Bradley Rea by divided decision in a outstanding all-Manchester battle to secure the continental championship.
The stadium grew hushed for a profoundly moving homage before the fight as ten bells tolled in remembrance of the city's native Hatton, who succumbed 46 years old in recently.
It was the first major televised boxing event in the city since Hatton's passing. As the final chime ended, the stillness transitioned to a shout of "only one Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was floored by a significant blow flush on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the Manchester United-red shorts of Arthur soon advanced again. On the opposite side, Rea - in rival City blue - was damaged soon, heavy swelling appearing under his face as the duo traded hard punches.
Rea, 27, landed power punches but the champion, 34, matched the intensity with effective uppercuts as the pace rarely dropped over twelve rounds.
Arthur barely secured the verdict with judges scorecards of even score, narrow lead and wider gap, but both fighters were relentless and courageous, exactly what their idol Hatton stood for.
