British light heavyweight Buatsi made a winning yet underwhelming start to his career under fresh promotional backing with a points victory over Parker in Manchester.
The Olympic bronze medallist's work rate appeared to earn him a split decision in a close 10-round bout at the boxing arena.
But the challenger found success, landing some telling clean blows in a contest where the styles didn't complement each other, with constant grabbing disrupting the action.
Two judges scored it 96-94 to the winner, with the final judge a 95-95.
"It wasn't my best performance but what defines champions? They win," Buatsi said in the ringside as he was greeted with disapproval from the audience.
The Londoner got back to winning after his first professional defeat - an epic battle with Callum Smith in last winter - to earn a twentieth professional win.
Some ringside observers thought Parker, thirty-one, deserved the decision but the British boxer suffered the second loss of his 28-match professional journey.
The Champion Earns Triumph in Uneventful Encounter
His career development has stalled in recent times through promotional disputes and long spells of inactivity.
Previously predicted as the future of UK boxing, he now stands trying to fulfil that early promise - possibly in what feels like a final roll of the dice - under new promotional backing.
He began cautiously, with the two boxers willing to be aggressive in a cagey early stages.
The challenger, whose only previous defeat came at super middleweight against his former opponent in two years ago after breaking his hand in the contest, was competing in only his second bout at the higher weight class.
By the fight's midpoint, Parker had made it competitive, creating difficulties for the favorite, who operated with a solid defense to edge the cleaner rounds.
The spectators' acceptance wore thin because of frequent holding, with Parker slipping to the canvas various instances during grappling.
Parker closed well, but many rounds were hard to judge. Nevertheless, previous titleholder the boxing analyst - commentating on Dazn - had it significantly in favor for Parker, scoring for Buatsi just only one stanza.
Buatsi advances, but with the presence of power puncher David Benavidez, and Eastern European fighter Dmitry Bivol ruling the division, the road to world glory - on this evidence - still appears challenging.
Hatton Memorial Motivates Local Rivalry Fight
In the secondary fight, Lyndon Arthur beat Bradley Rea by divided decision in a terrific Manchester vs Manchester battle to secure the continental title.
The arena fell silent for a deeply poignant homage prior to the bout as 10 bells sounded in memory of the city's native Ricky Hatton, who died at age 46 in September.
It was the initial significant broadcast match card in the city since the passing of Hatton. As the concluding sound disappeared, the stillness gave way to a shout of "there's just one Ricky Hatton".
The action inside the ropes matched the feeling. Arthur was floored by a significant blow squarely on the jaw in the initial stage, standing uncertainly to his footing on the eight count.
But the red-colored attire of Arthur soon pressed ahead again. On the opposite side, Rea - in rival City blue - was marked up early, heavy swelling developing under his orbital area as the duo traded punishing blows.
The challenger, 27, delivered effective strikes but the champion, 34, responded in kind with stinging uppercuts as the pace hardly diminished over twelve rounds.
The victor narrowly won the result with official scores of 114-114, close margin and wider gap, but each competitor were persistent and full of heart, all the qualities their hero Hatton represented.
