UK light-heavyweight Buatsi made a winning yet unimpressive start to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a close ten-round fight at the boxing arena.
But Parker found success, delivering effective individual punches in a fight where the techniques didn't complement each other, with constant grabbing disrupting the action.
The officials had it 96-94 in favor to the winner, with the final judge a even score.
"It wasn't my finest display but what do champions do? They prevail," the victor stated in the ring as he was greeted with negative reactions from the audience.
The British fighter got back to winning after his maiden career setback - an thrilling contest with Callum Smith in last winter - to secure a 20th career victory.
Several boxing experts thought Parker, thirty-one, deserved the decision but the Derbyshire fighter suffered the second defeat of his 28-match career.
Buatsi Earns Win in Dull Encounter
The fighter's advancement has stalled in recent years through management issues and long spells of inactivity.
Earlier forecast as the rising talent of the British fight scene, he now is positioned trying to achieve expectations - perhaps in what appears to be a final roll of the dice - under new promotional backing.
He started carefully, with the two boxers willing to take risks 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 in the contest, was competing in only his second appearance at light heavyweight.
By the halfway stage, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to barely take the better rounds.
The spectators' acceptance diminished during frequent holding, with the challenger slipping to the canvas multiple occasions during close exchanges.
Parker ended stronger, but multiple periods were hard to judge. Although, previous titleholder the boxing analyst - commentating on the broadcast network - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor moves on, but with the likes of power puncher Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still seems difficult.
Ricky Hatton Tribute Fuels Local Rivalry Fight
In the secondary fight, Lyndon Arthur defeated Bradley Rea by divided decision in a terrific Manchester vs Manchester fight to win the continental belt.
The arena became quiet for a extremely emotional memorial ahead of the contest as ten bells rang in memory of Manchester's own Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the urban center since Hatton's passing. As the final chime disappeared, the silence transitioned to a chant of "only one Hatton".
The action during the bout reflected the emotion. The eventual winner was sent to the canvas by a powerful punch squarely on the face in the second round, getting up unsteadily to his footing on the eight count.
But the traditional red equipment of the fighter soon came forward again. On the opposite side, the opponent - in opposing blue - was marked up quickly, significant bruising appearing under his eye as the pair traded hard punches.
The challenger, twenty-seven, connected with strong blows but Arthur, 34, fought fire with fire with stinging uppercuts as the pace seldom decreased over 12 rounds.
The victor barely secured the result with official scores of even score, close margin and clearer margin, but each competitor were relentless and full of heart, all the qualities their inspiration Hatton embodied.
