British light heavyweight Buatsi secured a victory yet unimpressive start to his career under new promoters with a points victory over Parker in the Manchester arena.
The Olympic bronze medallist's punch output likely gained him a majority decision in a competitive 10-round fight at the boxing arena.
But Parker had his moments, delivering effective single shots in a match where the styles never quite clicked, with frequent clinching disrupting the action.
The officials judged it 96-94 in favor to the winner, with the third a 95-95.
"It wasn't my finest display but what do champions do? They prevail," Buatsi commented in the ring as he was met by boos from the crowd.
The Londoner got back to winning after his initial pro loss - an epic battle with Callum Smith in February - to secure a 20th pro success.
Various analysts thought the challenger, thirty-one, deserved the decision but the British boxer suffered the second defeat of his 28-fight career.
The Champion Claims Win in Dull Match
His career development has stalled in recent times through contract problems and long spells of inactivity.
Once tipped as the future of the British fight scene, he now finds himself trying to achieve expectations - maybe in what feels like a ultimate opportunity - under new promotional backing.
He opened tentatively, with the two boxers prepared to be aggressive in a tentative opening few rounds.
The challenger, whose only previous defeat came at super-middleweight against John Ryder in two years ago after breaking his hand mid-fight, was competing in only his second appearance at light-heavyweight.
By the fight's midpoint, the underdog had made it competitive, creating difficulties for the favorite, who worked behind a tight guard to barely take the better periods.
The crowd's patience decreased because of frequent holding, with Parker going down several times during close exchanges.
Parker finished the stronger, but multiple periods were difficult to score. However, ex-champion the boxing analyst - commentating on Dazn - had it strongly leaning for Parker, scoring for Buatsi just one round.
The victor moves on, but with the likes of power puncher Benavidez, and Russian star Bivol dominating the weight class, the path to championship success - judging by this showing - still appears challenging.
Hatton Homage Inspires All-Manchester Fight
In the supporting bout, Lyndon Arthur beat Bradley Rea by close verdict in a outstanding all-Manchester fight to win the European light-heavyweight belt.
The stadium grew hushed for a extremely emotional memorial before the fight as a bell ceremony tolled in honor of local hero Hatton, who died at age 46 in recently.
It was the initial significant broadcast match card in the urban center since Hatton's passing. As the concluding sound disappeared, the quiet transitioned to a shout of "there's only one Hatton".
The fighting within the ring reflected the sentiment. The eventual winner was floored by a significant blow directly to the chin in the initial stage, rising shakily to his feet on the referee's count.
But the Manchester United-red equipment of the fighter soon pressed ahead again. Across the ring, Rea - in opposing blue - was marked up early, heavy swelling developing under his face as the fighters traded powerful shots.
The challenger, twenty-seven, landed power punches but the champion, 34, fought fire with fire with powerful upward punches as the intensity rarely dropped over 12 rounds.
The victor narrowly won the result with official scores of even score, narrow lead and clearer margin, but the two athletes were determined and full of heart, all the qualities their inspiration Hatton represented.
