UK light heavyweight Buatsi earned a win yet underwhelming debut to life under recent management with a unanimous decision over Parker in Manchester.
The Olympic bronze medallist's punch output appeared to earn him a majority decision in a close ten-round bout at the Manchester venue.
But the challenger found success, delivering effective clean blows in a match where the styles failed to mesh, with excessive holding spoiling the contest.
The scoring judges judged it 96-94 for to Buatsi, with the remaining official a even score.
"It wasn't my best display but how do champions respond? They prevail," Buatsi said in the arena as he was greeted with disapproval from the crowd.
The British fighter bounced back after his maiden career setback - an thrilling contest with Callum Smith in last winter - to claim a twentieth professional win.
Various analysts believed the challenger, thirty-one, merited the victory but the contender experienced the second defeat of his 28-fight professional journey.
Buatsi Claims Win in Slow Affair
His career development has plateaued in recent years through contract problems and extended periods of lack of fights.
Once tipped as the next star of British boxing, he now is positioned trying to fulfil that early promise - maybe in what appears to be a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to be aggressive in a cagey initial rounds.
Parker, whose only previous defeat came at super middleweight against his former opponent in 2022 after suffering a hand injury during the bout, was participating in his second bout at the higher weight class.
By the middle rounds, the underdog had kept it close, making life awkward for the favorite, who operated with a tight guard to narrowly win the cleaner periods.
The spectators' acceptance decreased amid regular grabbing, with Parker slipping to the canvas several times during close exchanges.
The underdog finished the stronger, but many rounds were difficult to score. Although, previous titleholder Carl Frampton - working with the broadcast network - had it strongly leaning for the challenger, awarding Buatsi just a single round.
The victor moves on, but with the caliber of power puncher David Benavidez, and Russian champion Dmitry Bivol leading the category, the road to world glory - judging by this showing - still appears challenging.
Hatton Tribute Fuels Manchester Derby Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by split-decision in a terrific Manchester vs Manchester contest to capture the European light-heavyweight championship.
The stadium fell silent for a profoundly moving homage ahead of the contest as ten bells rang in honor of the city's native Hatton, who succumbed 46 years old in recently.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the final chime ended, the stillness gave way to a shout of "there's only one Ricky Hatton".
The competition during the bout reflected the sentiment. Arthur was knocked down by a left hook directly to the face in the initial stage, rising shakily to his position on the eight count.
But the traditional red equipment of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was damaged quickly, noticeable damage appearing under his orbital area as the pair traded hard punches.
The challenger, 27, landed power punches but the champion, thirty-four, fought fire with fire with stinging uppercuts as the intensity seldom decreased over 12 rounds.
The victor narrowly won the decision with scoring tallies of 114-114, close margin and clearer margin, but each competitor were persistent and full of heart, all the qualities their hero the legend represented.
