UK light-heavyweight Buatsi earned a win yet disappointing debut to life under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a judges' verdict in a competitive 10-round fight at the boxing arena.
But the challenger landed significant shots, landing some telling clean blows in a fight where the styles didn't complement each other, with frequent clinching marring the action.
The officials had it 96-94 in favor to the winner, with the third a 95-95 draw.
"It wasn't my greatest display but what do champions do? They prevail," the victor commented in the ringside as he was greeted with boos from the crowd.
The British fighter got back to winning after his maiden career setback - an epic battle with his previous opponent in February - to claim a 20th career victory.
Some ringside observers believed the challenger, thirty-one, deserved the victory but the Derbyshire fighter experienced the second defeat of his 28-fight professional journey.
The Champion Secures Victory in Uneventful Encounter
Buatsi's progress has stalled in the past few years through contract problems and long spells of inactivity.
Earlier forecast as the next star of the British fight scene, he now is positioned trying to fulfil that early promise - maybe in what seems like a final roll of the dice - under recent support.
He opened tentatively, with the two boxers willing to overcommit in a tentative opening few rounds.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was participating in his second bout at light-heavyweight.
By the halfway stage, the underdog had made it competitive, creating difficulties for Buatsi, who operated with a tight guard to barely take the more effective periods.
The spectators' acceptance wore thin because of regular grabbing, with the challenger slipping to the canvas several times during grappling.
The underdog closed well, but multiple periods were hard to judge. However, previous titleholder Carl Frampton - providing analysis for the broadcast network - had it particularly wide for Parker, giving Buatsi just one round.
The victor advances, but with the likes of hard hitter David Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - on this evidence - still seems difficult.
Hatton Tribute Fuels Manchester Derby Fight
In the secondary fight, Arthur beat Bradley Rea by close verdict in a outstanding local rivalry contest to capture the European light-heavyweight title.
The arena became quiet for a extremely emotional homage ahead of the contest as 10 bells tolled in memory of local hero Ricky Hatton, who passed away aged 46 in September.
It was the first major televised fight night in the city since Hatton's death. As the last bell disappeared, the silence became a roar of "only one Ricky Hatton".
The fighting within the ring equaled the emotion. Arthur was sent to the canvas by a significant blow directly to the face in the early round, standing uncertainly to his footing on the eight count.
But the red-colored equipment of Arthur soon came forward again. From the other corner, the opponent - in opposing blue - was injured soon, heavy swelling appearing under his eye as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but Arthur, thirty-four, responded in kind with powerful upward punches as the intensity rarely dropped over the full distance.
Arthur narrowly won the result with official scores of even score, 115-113 and clearer margin, but the two athletes were persistent and courageous, all the qualities their inspiration Hatton embodied.
