British light-heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic medal winner's punch output likely gained him a majority decision in a competitive ten-round bout at the Manchester venue.
But Parker landed significant shots, connecting with powerful clean blows in a match where the approaches failed to mesh, with frequent clinching disrupting the action.
The officials had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but what defines champions? They succeed," the victor stated in the arena as he was faced boos from the audience.
The British fighter returned to winning ways after his first professional defeat - an epic battle with his previous opponent in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, 31, deserved the win but the Derbyshire fighter suffered the second defeat of his 28-bout professional journey.
Buatsi Secures Triumph in Uneventful Affair
Buatsi's progress has plateaued in recent times through contract problems and long spells of lack of fights.
Previously predicted as the future of UK boxing, he now finds himself trying to achieve expectations - perhaps in what appears to be a ultimate opportunity - under new promotional backing.
He started carefully, with both competitors ready to be aggressive in a tentative initial rounds.
Parker, whose single earlier setback came at the lower weight class against John Ryder in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at light-heavyweight.
By the halfway stage, the underdog had kept it close, creating difficulties for Buatsi, who worked behind a protective stance to barely take the better stanzas.
The crowd's patience decreased amid regular grabbing, with the challenger falling to the floor several times during clinches.
Parker closed well, but multiple periods were challenging to assess. Although, previous titleholder the boxing analyst - providing analysis for the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just only one stanza.
The victor advances, but with the presence of hard hitter Benavidez, and Russian star Bivol dominating the weight class, the journey to titles - based on this performance - still looks steep.
Ricky Hatton Memorial Inspires Local Rivalry Fight
In the supporting bout, Arthur defeated Rea by close verdict in a terrific all-Manchester fight to capture the European light-heavyweight title.
The venue fell silent for a deeply poignant tribute before the fight as a bell ceremony sounded in memory of Manchester's own Ricky Hatton, who succumbed 46 years old in last month.
It was the initial significant broadcast match card in the location since Hatton's passing. As the last bell faded, the silence became a roar of "there's just one Ricky Hatton".
The action within the ring matched the sentiment. Arthur was sent to the canvas by a powerful punch squarely on the chin in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged early, heavy swelling forming under his face as the pair landed hard punches.
The challenger, twenty-seven, delivered effective strikes but the champion, thirty-four, responded in kind with powerful upward punches as the action seldom decreased over twelve rounds.
Arthur edged the result with official scores of draw, narrow lead and wider gap, but both fighters were persistent and brave, exactly what their hero the legend represented.
