UK light heavyweight Buatsi earned a win yet underwhelming beginning to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's work rate seemingly secured him a majority decision in a competitive 10-round bout at the Manchester venue.
But the challenger found success, connecting with powerful single shots in a contest where the approaches never quite clicked, with excessive holding disrupting the fight.
The officials judged it 96-94 to Buatsi, with the third a 95-95.
"It wasn't my finest display but what defines champions? They prevail," the victor commented in the ring as he was greeted with disapproval from the crowd.
The Londoner bounced back after his maiden career setback - an epic battle with his previous opponent in February - to claim a twentieth professional win.
Several boxing experts believed the challenger, thirty-one, merited the victory but the contender experienced the second setback of his 28-bout career.
The Champion Earns Triumph in Dull Encounter
Buatsi's progress has stalled in recent times through contract problems and long spells of lack of fights.
Previously predicted as the future of British boxing, he now stands trying to realize his potential - possibly in what appears to be a final roll of the dice - under fresh management.
He started carefully, with neither fighter ready to take risks in a cagey initial rounds.
Parker, whose sole prior loss came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was making just his second fight at the higher weight class.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to edge the better periods.
The crowd's patience diminished amid frequent holding, with Parker going down various instances during close exchanges.
Parker ended stronger, but many rounds were hard to judge. Although, former world champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just a single round.
The victor advances, but with the likes of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Tribute Fuels Local Rivalry Battle
In the secondary fight, Arthur beat Bradley Rea by close verdict in a terrific local rivalry contest to win the European light-heavyweight championship.
The venue grew hushed for a extremely emotional tribute before the fight as 10 bells rang in memory of local hero Hatton, who succumbed aged 46 in recently.
It was the first major televised fight night in the city since Hatton's death. As the concluding sound disappeared, the quiet became a chant of "there's only one Hatton".
The action within the ring matched the feeling. The eventual winner was knocked down by a left hook squarely on the chin in the early round, standing uncertainly to his feet on the referee's count.
But the Manchester United-red attire of Arthur soon came forward again. Across the ring, the opponent - in opposing blue - was damaged quickly, noticeable damage forming under his eye as the fighters exchanged hard punches.
Rea, 27, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the intensity seldom decreased over 12 rounds.
The victor edged the result with scoring tallies of draw, close margin and wider gap, but both fighters were persistent and brave, exactly what their hero Hatton embodied.
