British light-heavyweight Buatsi earned a win yet unimpressive start to his career under recent management with a unanimous decision over Parker in Manchester.
The Olympic medal winner's activity level appeared to earn him a majority decision in a competitive 10-round bout at the Co-op Live Arena.
But Parker had his moments, delivering effective individual punches in a fight where the approaches never quite clicked, with frequent clinching 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 finest performance but how do champions respond? They succeed," Buatsi said in the ring as he was faced disapproval from the spectators.
The Londoner returned to winning ways after his maiden career setback - an epic battle with Callum Smith in February - to claim a twentieth professional win.
Various analysts felt Parker, 31, merited the decision but the contender suffered the second loss of his 28-fight professional journey.
The Champion Secures Triumph in Dull Encounter
His career development has stalled in recent years through management issues and long spells of inactivity.
Once tipped as the future of UK boxing, he now stands trying to fulfil that early promise - perhaps in what seems like a last chance - under recent support.
He started carefully, with neither fighter willing to take risks in a tentative opening few rounds.
The challenger, whose only previous defeat came at super middleweight against John Ryder in 2022 after breaking his hand in the contest, was competing in only his second appearance at light heavyweight.
By the fight's midpoint, Parker had stayed in contention, making life awkward for Buatsi, who operated with a tight guard to narrowly win the cleaner rounds.
The crowd's patience diminished during frequent holding, with the challenger falling to the floor multiple occasions during clinches.
The underdog finished the stronger, but multiple periods were challenging to assess. Although, previous titleholder Carl Frampton - commentating on the broadcast network - had it strongly leaning for Parker, scoring for Buatsi just one round.
Buatsi advances, but with the likes of heavy hitter David Benavidez, and Russian star Dmitry Bivol ruling the division, the path to championship success - judging by this showing - still appears challenging.
Ricky Hatton Homage Fuels Manchester Derby Battle
In the supporting bout, Lyndon Arthur defeated Rea by close verdict in a excellent all-Manchester contest to capture the European light-heavyweight championship.
The venue became quiet for a deeply poignant memorial before the fight as 10 bells rang in honor of the city's native Ricky Hatton, who died at age 46 in last month.
It was the initial significant broadcast fight night in the location since Hatton's passing. As the last bell ended, the silence transitioned to a roar of "there's just one Hatton".
The fighting inside the ropes reflected the emotion. The eventual winner was floored by a left hook squarely on the jaw in the second round, standing uncertainly to his position on the eight count.
But the Manchester United-red shorts of Arthur soon came forward again. Across the ring, the opponent - in competitive blue - was damaged quickly, heavy swelling appearing under his eye as the pair exchanged hard punches.
Rea, twenty-seven, connected with strong blows but the champion, 34, responded in kind with effective uppercuts as the intensity seldom decreased over twelve rounds.
The victor narrowly won the verdict with scoring tallies of 114-114, 115-113 and clearer margin, but both fighters were persistent and brave, everything their inspiration Hatton stood for.
