UK light-heavyweight Buatsi secured a victory yet unimpressive start to life under recent management with a points victory 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 boxing arena.
But the challenger found success, connecting with powerful single shots in a contest where the styles never quite clicked, with frequent clinching disrupting the fight.
The officials scored it 96-94 for to Buatsi, with the third a 95-95.
"It wasn't my finest performance but what defines champions? They win," Buatsi commented in the ringside as he was met by disapproval from the audience.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in February - to claim a 20th pro success.
Several boxing experts believed Parker, thirty-one, should have been awarded the victory but the Derbyshire fighter suffered the second defeat of his 28-fight career.
Buatsi Earns Triumph in Slow Encounter
The fighter's advancement has stalled in recent times through management issues and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now finds himself trying to fulfil that early promise - perhaps in what feels like a last chance - under fresh management.
He opened tentatively, with the two boxers prepared to overcommit in a calculated initial rounds.
The challenger, whose single earlier setback came at the lower weight class against John Ryder in two years ago after breaking his hand during the bout, was competing in only his second bout at light-heavyweight.
By the halfway stage, the underdog had stayed in contention, creating difficulties for the favorite, who worked behind a protective stance to barely take the cleaner stanzas.
The crowd's patience decreased during frequent holding, with Parker falling to the floor multiple occasions during grappling.
The underdog ended stronger, but many rounds were difficult to score. Although, ex-champion Carl Frampton - working with the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just a single round.
The victor advances, but with the presence of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - judging by this showing - still looks steep.
Hatton Memorial Fuels All-Manchester Fight
In the secondary fight, Arthur beat Bradley Rea by divided decision in a terrific local rivalry battle to secure the European light-heavyweight belt.
The arena grew hushed for a deeply poignant tribute before the fight as a bell ceremony sounded in honor of Manchester's own Hatton, who passed away aged 46 in recently.
It was the initial significant broadcast boxing event in the city since Hatton's death. As the final chime ended, the quiet became a shout of "only one Hatton".
The fighting inside the ropes matched the feeling. The eventual winner was floored by a powerful punch flush on the jaw in the early round, standing uncertainly to his footing on the count of eight.
But the traditional red equipment of Arthur soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged quickly, significant bruising forming under his orbital area as the fighters traded punishing blows.
The challenger, 27, delivered effective strikes but Arthur, 34, fought fire with fire with powerful upward punches as the intensity hardly diminished over 12 rounds.
Arthur edged the decision with official scores of draw, narrow lead and wider gap, but both fighters were persistent and brave, exactly what their inspiration the legend stood for.
