British light heavyweight Buatsi made a winning yet unimpressive debut to life under fresh promotional backing with a decision win over Parker in the Manchester arena.
The Olympic medal winner's work rate appeared to earn him a judges' verdict in a competitive 10-round fight at the Co-op Live Arena.
But Parker found success, delivering effective clean blows in a fight where the approaches never quite clicked, with constant grabbing marring the action.
The scoring judges had it 96-94 in favor to Buatsi, with the final judge a 95-95.
"It wasn't my finest display but how do champions respond? They prevail," the victor commented in the arena as he was greeted with boos from the crowd.
The Londoner bounced back after his initial pro loss - an thrilling contest with his previous opponent in February - to earn a twentieth professional win.
Several boxing experts felt Parker, thirty-one, deserved the decision but the contender endured the second defeat of his 28-bout career.
The Champion Secures Win in Dull Encounter
The fighter's advancement has slowed in recent years through management issues and extended periods of inactivity.
Once tipped as the rising talent of the British fight scene, he now finds himself trying to achieve expectations - maybe in what seems like a ultimate opportunity - under fresh management.
He started carefully, with the two boxers willing to be aggressive in a tentative initial rounds.
The challenger, whose sole prior loss came at super-middleweight against his former opponent in 2022 after breaking his hand during the bout, was competing in only his second fight at light heavyweight.
By the halfway stage, the underdog had kept it close, causing problems for Buatsi, who operated with a tight guard to narrowly win the cleaner rounds.
The crowd's patience decreased because of constant clinching, with the challenger going down various instances during clinches.
Parker finished the stronger, but many rounds were challenging to assess. Nevertheless, previous titleholder the boxing analyst - providing analysis for Dazn - had it strongly leaning for Parker, giving Buatsi just only one stanza.
Buatsi progresses, but with the caliber of power puncher David Benavidez, and Russian star Bivol leading the category, the road to world glory - judging by this showing - still looks steep.
Ricky Hatton Memorial Fuels Local Rivalry Fight
In the co-main event, Lyndon Arthur overcame Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to win the continental belt.
The stadium grew hushed for a profoundly moving homage ahead of the contest as 10 bells tolled in honor of the city's native Ricky Hatton, who succumbed at age 46 in September.
It was the premier televised boxing event in the location since Hatton's death. As the final chime ended, the quiet gave way to a chant of "there's only one Hatton".
The action during the bout equaled the feeling. The eventual winner was knocked down by a powerful punch directly to the face in the second round, getting up unsteadily to his footing on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was damaged soon, significant bruising developing under his eye as the pair landed punishing blows.
The challenger, twenty-seven, landed power punches but the champion, 34, responded in kind with effective uppercuts as the action hardly diminished over twelve rounds.
The victor edged the decision with official scores of draw, 115-113 and wider gap, but the two athletes were persistent and brave, all the qualities their idol the legend stood for.
