British light heavyweight Buatsi made a winning yet disappointing debut to his career under fresh promotional backing with a unanimous decision over Parker in Manchester.
The Olympic medal winner's work rate seemingly secured him a majority decision in a competitive 10-round fight at the Manchester venue.
But the challenger found success, landing some telling clean blows in a fight where the approaches never quite clicked, with constant grabbing marring the action.
Two judges scored it 96-94 to the winner, with the final judge a even score.
"It wasn't my finest display but how do champions respond? They win," Buatsi stated in the arena as he was met by disapproval from the audience.
The British fighter bounced back after his first professional defeat - an memorable fight with his previous opponent in last winter - to secure a twentieth professional win.
Several boxing experts felt the challenger, thirty-one, should have been awarded the win but the Derbyshire fighter endured the second defeat of his 28-match career.
Buatsi Claims Win in Slow Match
Buatsi's progress has stalled in recent years through management issues and extended periods of inactivity.
Earlier forecast as the rising talent of UK boxing, he now is positioned trying to achieve expectations - perhaps in what feels like a ultimate opportunity - under fresh management.
He opened tentatively, with both competitors prepared to be aggressive in a tentative early stages.
Parker, whose only previous defeat came at the lower weight class against his former opponent in 2022 after suffering a hand injury during the bout, was competing in only his second appearance at the higher weight class.
By the fight's midpoint, Parker had stayed in contention, causing problems for the favorite, who utilized a tight guard to narrowly win the more effective periods.
The spectators' acceptance wore thin because of frequent holding, with the challenger slipping to the canvas multiple occasions during clinches.
Parker finished the stronger, but several stanzas were challenging to assess. Although, former world champion the boxing analyst - 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 likes of heavy hitter Benavidez, and Eastern European fighter Dmitry Bivol leading the category, the journey to titles - judging by this showing - still appears challenging.
Hatton Tribute Motivates Manchester Derby Battle
In the secondary fight, Lyndon Arthur overcame Bradley Rea by divided decision in a excellent local rivalry battle to win the European light heavyweight championship.
The arena became quiet for a deeply poignant memorial prior to the bout as a bell ceremony rang in memory of local hero Hatton, who succumbed 46 years old in September.
It was the first major televised fight night in the urban center since Hatton's death. As the final chime ended, the quiet gave way to a roar of "there's only one Ricky Hatton".
The fighting inside the ropes equaled the sentiment. Arthur was knocked down by a left hook flush on the jaw in the second round, standing uncertainly to his position on the count of eight.
But the Manchester United-red equipment of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was injured early, significant bruising developing under his orbital area as the pair landed punishing blows.
The challenger, 27, landed power punches but the champion, 34, fought fire with fire with stinging uppercuts as the intensity hardly diminished over 12 rounds.
The victor barely secured the verdict with official scores of draw, narrow lead and wider gap, but the two athletes were persistent and courageous, exactly what their idol Hatton embodied.
