UK light-heavyweight Buatsi made a winning yet disappointing start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a judges' verdict in a close ten-round contest at the Manchester venue.
But Parker had his moments, connecting with powerful clean blows in a contest where the approaches didn't complement each other, with constant grabbing spoiling the contest.
Two judges judged it 96-94 in favor to the winner, with the remaining official a even score.
"It wasn't my greatest showing but what defines champions? They win," Buatsi said in the ring as he was greeted with boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an thrilling contest with Callum Smith in last winter - to earn a 20th pro success.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer endured the second setback of his 28-bout professional journey.
Buatsi Secures Victory in Uneventful Encounter
The fighter's advancement has stalled in recent years through management issues and extended periods of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under recent support.
He began cautiously, with neither fighter ready to take risks in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after suffering a hand injury mid-fight, was making just his second fight at light-heavyweight.
By the fight's midpoint, the underdog had stayed in contention, making life awkward for the favorite, who operated with a protective stance to narrowly win the better stanzas.
The audience's tolerance decreased because of regular grabbing, with the challenger falling to the floor multiple occasions during clinches.
The underdog closed well, but several stanzas were challenging to assess. Nevertheless, ex-champion Carl Frampton - working with the broadcast network - had it particularly wide for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the caliber of hard hitter David Benavidez, and Eastern European fighter Bivol dominating the weight class, the path to championship success - judging by this showing - still seems difficult.
Ricky Hatton Tribute Inspires All-Manchester Contest
In the co-main event, Arthur beat Rea by close verdict in a terrific all-Manchester fight to capture the European light-heavyweight championship.
The venue became quiet for a profoundly moving memorial ahead of the contest as 10 bells tolled in remembrance of the city's native Ricky Hatton, who died aged 46 in September.
It was the premier televised match card in the urban center since Hatton's passing. As the last bell ended, the stillness became a shout of "there's only one Ricky Hatton".
The competition inside the ropes matched the sentiment. The eventual winner was knocked down by a left hook flush on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon pressed ahead again. From the other corner, the opponent - in rival City blue - was marked up early, significant bruising developing under his eye as the duo landed powerful shots.
The challenger, twenty-seven, delivered effective strikes but Arthur, 34, matched the intensity with powerful upward punches as the pace seldom decreased over twelve rounds.
The victor barely secured the result with scoring tallies of 114-114, narrow lead and wider gap, but the two athletes were relentless and courageous, exactly what their hero Hatton stood for.
