UK light heavyweight Buatsi made a winning yet unimpressive debut to his career under new promoters with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level appeared to earn him a judges' verdict in a competitive 10-round bout at the Co-op Live Arena.
But the challenger landed significant shots, delivering effective single shots in a match where the techniques failed to mesh, with constant grabbing disrupting the action.
The scoring judges had it 96-94 to the winner, with the final judge a 95-95 draw.
"It wasn't my best display but what defines champions? They prevail," Buatsi stated in the arena as he was faced boos from the spectators.
The Londoner returned to winning ways after his initial pro loss - an memorable fight with his previous opponent in last winter - to secure a 20th pro success.
Some ringside observers believed Parker, 31, should have been awarded the win but the contender endured the second loss of his 28-bout career.
Buatsi Secures Victory in Dull Match
Buatsi's progress has plateaued in recent years through management issues and long spells of lack of fights.
Earlier forecast as the rising talent of the British fight scene, he now stands trying to realize his potential - possibly in what feels like a last chance - under fresh management.
He began cautiously, with neither fighter ready to be aggressive in a calculated early stages.
The challenger, whose single earlier setback came at super middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second appearance at light heavyweight.
By the middle rounds, the underdog had kept it close, making life awkward for Buatsi, who utilized a protective stance to barely take the cleaner periods.
The audience's tolerance wore thin amid regular grabbing, with the challenger slipping to the canvas multiple occasions during close exchanges.
The underdog ended stronger, but many rounds were difficult to score. Although, ex-champion the boxing analyst - providing analysis for the broadcast network - had it significantly in favor for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the likes of hard hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the road to world glory - on this evidence - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Contest
In the supporting bout, Arthur defeated Rea by close verdict in a excellent all-Manchester fight to secure the continental title.
The venue became quiet for a deeply poignant tribute prior to the bout as a bell ceremony tolled in remembrance of the city's native Ricky Hatton, who succumbed at age 46 in recently.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the last bell ended, the stillness gave way to a roar of "only one Hatton".
The competition inside the ropes reflected the feeling. The eventual winner was floored by a left hook directly to the chin in the initial stage, standing uncertainly to his feet on the referee's count.
But the Manchester United-red equipment of Arthur soon came forward again. On the opposite side, Rea - in competitive blue - was injured early, noticeable damage forming under his eye as the pair landed powerful shots.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with effective uppercuts as the pace seldom decreased over twelve rounds.
Arthur edged the decision with judges scorecards of 114-114, 115-113 and clearer margin, but each competitor were persistent and brave, all the qualities their hero Hatton embodied.
