UK light-heavyweight Buatsi secured a victory yet disappointing start to life under new promoters with a decision win over Parker in Manchester.
The Olympic medal winner's work rate appeared to earn him a majority decision in a close ten-round bout at the boxing arena.
But Parker landed significant shots, delivering effective single shots in a match where the styles didn't complement each other, with constant grabbing marring the fight.
The scoring judges had it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best display but what do champions do? They prevail," Buatsi commented in the ring as he was faced disapproval from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Various analysts believed the challenger, thirty-one, merited the decision but the Derbyshire fighter endured the second loss of his 28-match career.
The Champion Claims Triumph in Uneventful Affair
Buatsi's progress has slowed in recent times through promotional disputes and long spells of inactivity.
Once tipped as the rising talent of UK boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a final roll of the dice - under new promotional backing.
He opened tentatively, with both competitors prepared to take risks in a cagey opening few rounds.
Parker, whose single earlier setback 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 stayed in contention, making life awkward for the favorite, who operated with a protective stance to barely take the better periods.
The spectators' acceptance diminished during regular grabbing, with the challenger falling to the floor several times during close exchanges.
Parker closed well, but multiple periods were difficult to score. However, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for the challenger, scoring for Buatsi just only one stanza.
Buatsi moves on, but with the likes of heavy hitter David Benavidez, and Eastern European fighter Bivol ruling the division, the journey to titles - based on this performance - still looks steep.
Hatton Homage Inspires All-Manchester Battle
In the co-main event, Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry contest to secure the continental championship.
The venue fell silent for a profoundly moving homage prior to the bout as 10 bells sounded in memory of Manchester's own Ricky Hatton, who succumbed aged 46 in September.
It was the first major televised boxing event in the location since Hatton's death. As the last bell disappeared, the stillness gave way to a roar of "only one Hatton".
The action during the bout reflected the emotion. Arthur was knocked down by a powerful punch directly to the face in the initial stage, standing uncertainly to his footing on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. On the opposite side, Rea - in opposing blue - was marked up quickly, noticeable damage forming under his orbital area as the pair landed powerful shots.
Rea, 27, connected with strong blows but Arthur, thirty-four, fought fire with fire with powerful upward punches as the intensity seldom decreased over twelve rounds.
The victor barely secured the result with scoring tallies of 114-114, close margin and wider gap, but each competitor were persistent and courageous, everything their idol the legend represented.
