British light-heavyweight Buatsi earned a win yet unimpressive start to life under recent management with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's work rate appeared to earn him a majority decision in a tight 10-round fight at the boxing arena.
But Parker found success, landing some telling clean blows in a contest where the styles never quite clicked, with constant grabbing disrupting the fight.
Two judges scored it 96-94 in favor to Buatsi, with the third a even score.
"It wasn't my best performance but how do champions respond? They succeed," Buatsi commented in the ringside as he was greeted with negative reactions from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with Callum Smith in February - to earn a twentieth professional win.
Several boxing experts felt Parker, 31, deserved the decision but the contender endured the second loss of his 28-match professional journey.
The Champion Earns Triumph in Slow Affair
His career development has stalled in recent times through contract problems and considerable time of lack of fights.
Previously predicted as the rising talent of UK boxing, he now stands trying to realize his potential - maybe in what appears to be a ultimate opportunity - under recent support.
He opened tentatively, with neither fighter willing to take risks in a tentative initial rounds.
Parker, whose single earlier setback came at super-middleweight against his former opponent in 2022 after breaking his hand mid-fight, was competing in only his second bout at light heavyweight.
By the middle rounds, Parker had kept it close, creating difficulties for the favorite, who operated with a protective stance to barely take the more effective rounds.
The audience's tolerance diminished amid frequent holding, with the challenger falling to the floor multiple occasions during close exchanges.
The underdog ended stronger, but multiple periods were challenging to assess. However, ex-champion the boxing analyst - providing analysis for the broadcast network - had it particularly wide for the challenger, scoring for Buatsi just one round.
The victor advances, but with the presence of heavy hitter David Benavidez, and Eastern European fighter Bivol leading the category, the journey to titles - based on this performance - still looks steep.
Hatton Memorial Fuels Local Rivalry Contest
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a outstanding local rivalry battle to win the continental title.
The stadium grew hushed for a extremely emotional tribute prior to the bout as a bell ceremony sounded in remembrance of the city's native Ricky Hatton, who died at age 46 in recently.
It was the first major televised boxing event in the urban center since the passing of Hatton. As the final chime ended, the quiet became a roar of "there's only one Hatton".
The action during the bout matched the sentiment. The eventual winner was knocked down by a left hook directly to the jaw in the early round, getting up unsteadily to his footing on the eight count.
But the red-colored shorts of Arthur soon came forward again. Across the ring, Rea - in opposing blue - was injured early, heavy swelling appearing under his eye as the duo exchanged hard punches.
Rea, twenty-seven, delivered effective strikes but Arthur, 34, fought fire with fire with effective uppercuts as the action seldom decreased over the full distance.
The victor narrowly won the verdict with official scores of 114-114, 115-113 and clearer margin, but both fighters were persistent and full of heart, all the qualities their hero Hatton represented.
