UK light heavyweight Buatsi secured a victory yet underwhelming debut to life under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's activity level likely gained him a majority decision in a competitive ten-round contest at the Manchester venue.
But Parker had his moments, landing some telling clean blows in a contest where the styles didn't complement each other, with excessive holding spoiling the action.
Two judges had it 96-94 to the winner, with the remaining official a even score.
"It wasn't my greatest display but what do champions do? They prevail," Buatsi stated in the arena as he was met by negative reactions from the crowd.
The British fighter returned to winning ways after his first professional defeat - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Various analysts felt Parker, 31, merited the decision but the Derbyshire fighter experienced the second defeat of his 28-bout professional journey.
The Champion Claims Win in Slow Affair
The fighter's advancement has stalled in recent years through promotional disputes and considerable time of inactivity.
Previously predicted as the rising talent of the British fight scene, he now finds himself trying to fulfil that early promise - maybe in what feels like a ultimate opportunity - under recent support.
He started carefully, with neither fighter prepared to take risks in a calculated early stages.
The challenger, whose single earlier setback came at super-middleweight against his former opponent in 2022 after damaging his hand in the contest, was participating in his second bout at light heavyweight.
By the middle rounds, Parker had stayed in contention, making life awkward for the favorite, who worked behind a tight guard to barely take the more effective periods.
The spectators' acceptance diminished amid constant clinching, with the challenger slipping to the canvas multiple occasions during clinches.
The underdog finished the stronger, but many rounds were difficult to score. Nevertheless, former world champion the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, giving Buatsi just a single round.
The victor moves on, but with the presence of hard hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the path to championship success - on this evidence - still looks steep.
Hatton Homage Fuels All-Manchester Fight
In the supporting bout, Lyndon Arthur defeated Bradley Rea by divided decision in a outstanding local rivalry contest to capture the continental title.
The stadium grew hushed for a deeply poignant memorial ahead of the contest as 10 bells tolled in remembrance of the city's native Hatton, who succumbed aged 46 in last month.
It was the initial significant broadcast boxing event in the location since Hatton's passing. As the last bell ended, the quiet transitioned to a chant of "only one Ricky Hatton".
The fighting within the ring equaled the sentiment. Arthur was knocked down by a significant blow squarely on the jaw in the initial stage, standing uncertainly to his feet on the referee's count.
But the red-colored shorts of the fighter soon advanced again. On the opposite side, the opponent - in competitive blue - was damaged early, heavy swelling developing under his orbital area as the pair traded powerful shots.
Rea, twenty-seven, delivered effective strikes but Arthur, thirty-four, responded in kind with stinging uppercuts as the pace seldom decreased over twelve rounds.
The victor narrowly won the verdict with scoring tallies of draw, close margin and clearer margin, but the two athletes were determined and brave, all the qualities their idol Hatton stood for.
