British light-heavyweight Buatsi earned a win yet unimpressive beginning to his career under fresh promotional backing with a unanimous decision over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive 10-round contest at the Manchester venue.
But Parker had his moments, delivering effective single shots in a fight where the techniques didn't complement each other, with excessive holding disrupting the fight.
The scoring judges judged it 96-94 for to the winner, with the remaining official a even score.
"It wasn't my greatest performance but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with boos from the audience.
The British fighter bounced back after his maiden career setback - an memorable fight with Callum Smith in last winter - to secure a 20th career victory.
Some ringside observers believed Parker, thirty-one, should have been awarded the win but the British boxer experienced the second setback of his 28-fight professional journey.
The Champion Earns Triumph in Slow Encounter
The fighter's advancement has plateaued in the past few years through promotional disputes and extended periods of inactivity.
Previously predicted as the rising talent of the British fight scene, he now stands trying to fulfil that early promise - possibly in what seems like a last chance - under recent support.
He started carefully, with neither fighter prepared to overcommit in a cagey opening few rounds.
Parker, whose sole prior loss came at super-middleweight against his former opponent in 2022 after suffering a hand injury mid-fight, was making just his second appearance at light heavyweight.
By the middle rounds, Parker had made it competitive, making life awkward for the favorite, who worked behind a protective stance to edge the better periods.
The audience's tolerance wore thin because of frequent holding, with Parker going down multiple occasions during grappling.
Parker ended stronger, but many rounds were challenging to assess. Nevertheless, former world champion Carl Frampton - commentating on Dazn - had it particularly wide for the challenger, giving Buatsi just a single round.
The victor progresses, but with the likes of heavy hitter Benavidez, and Russian champion Bivol dominating the weight class, the path to championship success - on this evidence - still looks steep.
Hatton Tribute Motivates Local Rivalry Battle
In the co-main event, Lyndon Arthur overcame Bradley Rea by split-decision in a outstanding local rivalry battle to capture the continental title.
The arena fell silent for a extremely emotional tribute prior to the bout as 10 bells tolled in remembrance of local hero Ricky Hatton, who died at age 46 in last month.
It was the premier televised fight night in the city since Hatton's death. As the last bell faded, the quiet transitioned to a chant of "only one Hatton".
The action during the bout reflected the emotion. The eventual winner was knocked down by a significant blow directly to the jaw in the initial stage, getting up unsteadily to his feet on the referee's count.
But the red-colored equipment of the fighter soon came forward again. Across the ring, Rea - in rival City blue - was damaged soon, significant bruising appearing under his face as the duo landed hard punches.
Rea, twenty-seven, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the action rarely dropped over the full distance.
The victor barely secured the decision with judges scorecards of even score, 115-113 and wider gap, but both fighters were persistent and brave, everything their idol Hatton represented.
